How to Dose Amoxicillin (Fish Mox) in Aquariums by Gallon

Fish Mox (Amoxicillin) is a broad-spectrum aquarium antibiotic used by hobbyists to treat common bacterial fish diseases. Dosing it correctly by aquarium volume (gallons) is critical for effective treatment and fish safety. This guide will explain how to dose Fish Mox by gallon using 250 mg and 500 mg capsules, provide a handy dosing chart for different tank sizes, and outline the treatment schedule (re-dosing intervals and water changes). We’ll also cover which fish illnesses amoxicillin treats – such as fin rot, columnaris, “popeye,” mouth fungus, and other infections – and how to properly administer the medication (like dissolving capsules for even distribution). Finally, we’ll highlight where to buy Fish Mox without a prescription (legally in the U.S.) and mention alternative fish antibiotics available for aquarium use.

What Is Fish Mox (Amoxicillin) and Why Use It?

Fish Mox is the trade name for amoxicillin formulated for fish. It’s an antibiotic capsule (usually 250 mg or 500 mg) that can be opened and added to aquarium water to treat bacterial infections. Amoxicillin is a bactericidal antibiotic effective against many gram-positive and some gram-negative bacteria that affect fish​. In aquariums, Fish Mox is valued for its broad spectrum of activity and relative safety for most fish when used correctly. It’s commonly used in both freshwater and saltwater hobby aquariums to address infections that commercial fish medications might not fully cure.

Why use Fish Mox? It provides a targeted treatment for bacterial outbreaks in fish without needing a vet prescription. For U.S. aquarists, Fish Mox is available over-the-counter specifically for ornamental fish use (not for animals intended for human consumption). This makes it convenient when your fish show signs of a bacterial infection and you need a fast remedy. When dosed properly, amoxicillin can help save fish by clearing infections like fin rot or columnaris that would otherwise progress. Always ensure you’re using Fish Mox for the right illness and at the correct dosage to avoid under-dosing (which might not cure the infection) or overdosing (which could stress fish or damage the aquarium’s bio-filter).

Fish Diseases Treated by Amoxicillin (Fish Mox)

One reason Fish Mox (amoxicillin) is popular is the range of common fish diseases it can treat. It’s effective against many external bacterial infections in fish. Here are some illnesses and conditions where amoxicillin is often used:

• Fin Rot: A bacterial infection that causes fins to fray, rot, or discolor. Amoxicillin can help eliminate the bacteria (e.g. Aeromonas or Pseudomonas) causing fin and tail rot​, allowing the fins to heal.

• Columnaris (Cotton Mouth/Mouth Fungus): A Flavobacterium infection often called “cotton wool disease” or “mouth fungus,” which appears as white fuzzy patches on the mouth, fins, or body. Despite its nickname, it’s bacterial, not a true fungus. Amoxicillin is one treatment option to combat columnaris​, helping to clear the infection.

• Popeye (Exophthalmia): A condition where one or both fish eyes swell and bulge out, often due to bacterial infection behind the eye. Fish Mox can treat the underlying infection causing popeye​, reducing inflammation over a course of treatment.

• Mouth Rot/Fungus: Often related to columnaris, this appears as mold-like growth around the mouth or lips. Amoxicillin’s broad action on gram-positive bacteria may help in treating these “mouth rot” cases, especially if caused by columnaris bacteria.

• Gill Infections: Bacterial gill disease (e.g. Mycobacterium or Chondrococcus) can cause swollen, discolored gills and rapid breathing. Amoxicillin is indicated for some gill infections​, improving gill function as the bacteria are cleared.

• General Bacterial Infections: Fish Mox can also treat other bacterial issues like skin ulcers, open wounds, infected eyes, or body slime infections​. If your fish has red sores or cloudy eyes from a bacterial cause, amoxicillin may be prescribed in the aquarium to help cure it.

Important: Amoxicillin is most effective on bacterial infections. It will not treat parasitic diseases (like ich or flukes) or true fungal infections. Always diagnose as best as possible – if unsure, consult a knowledgeable aquarist or aquatic veterinarian. Signs like frayed fins, white fuzzy growths, bulging eyes, or red ulcers are typically bacterial and can respond to Fish Mox.

Dosage Guidelines: mg Per Gallon of Aquarium Water

When treating fish with amoxicillin, dosing is usually based on the aquarium’s water volume. The general recommendation is to dose between 10–20 mg of amoxicillin per gallon of water, depending on the severity of the infection. Using the right amount is crucial: too little may be ineffective, and too much can stress fish or harm beneficial bacteria in the tank.

• Standard Dosage Range: 10–20 mg per gallon. This means for each 1 gallon of water, use between 10 and 20 milligrams of amoxicillin. For example, a mild infection in a 10-gallon tank might use ~100 mg per dose (10 mg×10 gal), while a severe infection might use up to ~200 mg (20 mg×10 gal) per dose.

• Understanding Capsule Strengths: Fish Mox typically comes in 250 mg capsules (regular strength) and 500 mg capsules (often labeled “Fish Mox Forte”). Each capsule contains that amount of amoxicillin powder. You may need to use whole or partial capsules to achieve the correct mg-per-gallon dosage for your tank size:

  • A 250 mg capsule can treat roughly 12 to 25 gallons of water (since 250 mg divided by 12 gal ≈20 mg/gal and divided by 25 gal =10 mg/gal). In practice, many hobbyists simplify this to 1 capsule per 10 gallons, which is a slightly stronger dose (~25 mg/gal)​ but within a safe range for serious infections.

  • A 500 mg capsule treats about 25 to 50 gallons (500 mg/25 gal = 20 mg/gal; 500 mg/50 gal = 10 mg/gal). This often translates to 1 capsule per 20 gallons as a convenient rule (again about 25 mg/gal)​.

• Adjusting for Your Tank: It’s best to calculate based on your exact water volume. Remember to account for decor, substrate, etc., which displace water – your “net” gallons might be a bit less than the tank’s rated size. When in doubt, err on the side of slightly under-dosing (e.g., use 10–15 mg/gal) rather than grossly overdosing, especially if the infection is not life-threatening.

Most manufacturers’ instructions for Fish Mox simplify dosing as mentioned: 250 mg per 10 gallons (or 500 mg per 20 gallons), administered each day​. We’ll provide a detailed chart below for common tank sizes using both capsule sizes. Always use milligrams (mg) per gallon as your basis, and you can scale up or down for any aquarium volume.

Fish Mox Dosage Chart by Tank Size

To make dosing even easier, use this chart for common aquarium sizes. It shows the approximate amount of amoxicillin needed (in mg) and how that translates into 250 mg or 500 mg Fish Mox capsules:

How to use this chart: Find your aquarium’s size in gallons in the first column. The second and third columns show the range of total milligrams of amoxicillin to aim for (low end at 10 mg/gal, high end at 20 mg/gal). The last two columns show approximately how many capsules of Fish Mox you’d use to reach that dose, whether you have 250 mg capsules or 500 mg capsules.

For small tanks like 5 or 10 gallons, you will be using portions of a capsule. You must open the capsule and dissolve part of the powder (don’t just drop a whole capsule in a tiny tank!). For example, a 5-gallon tank might need ~75 mg (middle of the range), which is about ⅓ of a 250 mg capsule. You’d open the capsule and add roughly one-third of the powder to the water (discard or save the rest for the next dose). If you only have 500 mg capsules on hand for a small tank, it’s better to mix a full capsule into a measured amount of water and dose a fraction of that solution to be more precise.

For larger tanks (20+ gallons), it’s easier: one 250 mg cap per 10 gal or one 500 mg cap per 20 gal is a convenient rule of thumb. For example, a 40-gallon aquarium would get 4×250 mg capsules or 2×500 mg capsules daily. This matches the common recommendations (500 mg for 20 gallons, 5 capsules for 100 gal, etc.​).

Keep in mind that the “mg per gallon” approach is a guideline. Hitting the exact number isn’t as critical as staying in the therapeutic range. Fish Mox is fairly forgiving, but always stay within sensible limits (around 10–25 mg/gal). When in doubt, start with a moderate dose (~15 mg/gal) and you can adjust in the next dose if needed. It’s better to slightly under-dose than overdose on the first day if you’re unsure of your exact water volume or your fish’s sensitivity.

Treatment Schedule: Frequency and Duration of Dosing

Dosing Fish Mox is not a one-time event – you’ll need to dose repeatedly over several days to fully eradicate the infection. The typical treatment schedule for amoxicillin in aquariums is:

• Dose Every 24 Hours: Amoxicillin breaks down in the water and loses effectiveness after about a day. Plan to add the calculated dose once per day (every 24 hours). Try to dose at the same time each day for consistency. For example, if you dose at 7 PM on day 1, do the same on day 2, etc.

• Treatment Duration – 5 to 10 Days: A full course is usually at least 5 days, and up to 10 days of daily dosing​. For mild infections, 5–7 days may suffice. For more severe or stubborn infections (like advanced fin rot or columnaris), continue for 10 days. Do not exceed 10 days of continuous treatment with amoxicillin, as this can stress fish and bio-filters. If there’s no improvement after 5 days, it’s recommended to re-evaluate the treatment – you might need a different antibiotic or approach​.

• Observe Progress: By day 3-5, you should see some improvement (e.g. less fin deterioration, reduced fungus-like patches, fish becoming more active). If the condition worsens or there’s zero improvement after several days, consider consulting an expert or switching medication. But do not stop treatment too early if the fish is improving – complete the full course (5 days minimum) to ensure the infection is truly gone, much like finishing a course of antibiotics for people.

• After the Treatment Course: Once you’ve dosed for the prescribed days (5-10 days), stop dosing and do not continue adding more antibiotic. At the end of treatment, perform a larger water change (more on this below) and consider adding activated carbon to start removing any remaining medication from the water. It’s also a good practice to monitor your fish for the next week after treatment; if symptoms recur, you may need a second round or a different medication.

Note: One manufacturer’s instructions indicated a 5-day single-dose treatment for corals (with one dose lasting 5 days)​, but for fish in tank water the standard approach is daily dosing. Always follow the regimen that corresponds to treating fish in an aquarium environment – generally the every-24-hours schedule unless directed otherwise by a vet or specific product instructions.

Water Changes During Antibiotic Treatment

Maintaining water quality is always important in an aquarium antibiotic treatment. When using Fish Mox, you should plan for periodic water changes during the course of treatment. Here’s how to manage water changes without reducing the medication’s effectiveness:

• Partial Water Change Between Doses: It’s often recommended to perform a partial water change (about 20–25%) before each daily dose​. For example, every 24 hours, just before re-dosing, remove 20-25% of the tank water and replace it with fresh dechlorinated water. This helps in a few ways: it reduces waste build-up (since antibiotics can suppress the filter bacteria), ensures the water stays oxygenated and clean for the stressed fish, and removes some of the spent medication from the previous day. Don’t worry about removing too much medication with a 20% change – you will be adding a full new dose right after the water change.

• Coordinate With Dosing Time: Do the water change immediately before dosing the next capsule. That way, the fresh water doesn’t dilute an active dose that’s still working. Essentially, each day you reset the water quality a bit, then add the new dose which will be at full strength for the next 24 hours.

• Maintain Consistent Treatment Levels: Amoxicillin can impart a slight yellow tint to the water over multiple days. This is harmless, but you’ll notice the color. By doing partial water changes, you prevent the discoloration from becoming too strong and keep medication levels more consistent. (The yellow hue is normal; the product literature notes it can be removed later with carbon filtration​.)

• After Completing Treatment – Larger Water Change: Once the treatment course is finished (day 5, 7, or 10, etc.), do a more substantial water change, such as 40–50%. This will dilute the remaining antibiotic. Right after that, you can place activated carbon in your filter (if you use a hang-on-back or canister filter) or a carbon filter pad to absorb residual medication. Within a day or two, the carbon should clear out the rest of the amoxicillin and any yellow tint​. You can also resume use of any chemical filtration media (like Purigen) that you had removed for treatment.

• Minimize Stress During Water Changes: Ensure the new water is dechlorinated, temperature-matched, and roughly the same pH as the tank to avoid shocking your fish. Gentle vacuuming of the substrate can remove decaying matter that could feed bacteria. The goal is to improve water quality without causing additional stress to sick fish.

Tip: While treating, turn off UV sterilizers and remove activated carbon from your filter at the start of treatment​. UV light can break down certain medications, and carbon will actively strip amoxicillin out of the water (undoing your dosing work!). So disable those until you’re finished dosing. You can keep mechanical and biological filtration running to maintain oxygenation and some filtration (though be aware the antibiotic may affect your biofilter bacteria). After treatment, carbon can be used as noted to clean the water.

How to Dissolve Fish Mox Capsules for Even Dosing

When adding Fish Mox to your aquarium, never just toss the closed capsule into the tank – it may not dissolve evenly, and the powder could clump or not disperse properly. The correct method is to open the capsule and pre-dissolve the powder before introduction. This ensures the medication mixes uniformly in the water so all fish receive the treatment. Follow these steps for dissolving Fish Mox capsules:

1. Wash Your Hands and Prep a Container: Start with clean hands and use a clean container (like a disposable cup or jar dedicated for aquarium use). Draw some aquarium water into the container – enough to easily mix (about 1 cup or more).

2. Open the Capsule Carefully: Holding the Fish Mox capsule upright, gently twist or pull apart the two halves over a piece of paper or the container. The powder inside is the amoxicillin. (If you only need a portion of the capsule, you might use a fine clean knife or spatula to divide the powder – since it’s a fine powder, an approximate division is usually okay.)

3. Measure or Divide Powder if Needed: If your dose is not a full capsule, you can split the powder. One way is to empty the entire capsule onto a clean paper and, using a razor blade or card, divide it into equal piles (for example, roughly halves, quarters, etc. to get the portion you need). Another method is dissolving the full capsule in a known volume of water (say 500 ml) and then only dosing a proportional volume of that solution (e.g., 250 ml for half the dose). Most hobbyists will eyeball portions carefully – precision to the milligram isn’t critical as long as you’re within the dosing range.

4. Mix with Aquarium Water: Whether you have the full capsule’s powder or a portion, add the powder into the cup of aquarium water. Stir it or cap and shake the container to mix until the powder dissolves as much as possible​. Amoxicillin powder should dissolve reasonably well; some fillers might remain as sediment, which is fine.

5. Pour Into the Aquarium: Slowly pour the dissolved medication into a high-flow area of the aquarium​. Good spots are in front of a filter return, powerhead, or air stone so that the solution disperses quickly. This prevents the antibiotic from settling in one spot. You might even pour in multiple spots for even distribution.

6. Ensure Even Distribution: Watch the water flow carry the medication around. You shouldn’t see clumps of powder—if you do, gently stir the water or use a turkey baster to blow the clump into the flow to help it dissolve. Within a few minutes, the medication will be thoroughly mixed in the tank water.

By pre-dissolving the capsules, you avoid any hot spots of concentration that could irritate fish (for instance, if a fish tried to nibble on a chunk of undissolved powder, that could be harmful). It also guarantees that the dose is evenly spread so all areas of the tank and all fish are exposed to the antibiotic.

Safety notes while dosing: It’s a good idea to wear disposable gloves when handling antibiotics, and wash up after. Though Fish Mox is marketed for fish, it’s still amoxicillin – people who are allergic to penicillin-type drugs should be cautious not to touch the powder​. Also, keep the capsules and powder away from children and pets. Only use Fish Mox for aquarium fish; do not use it for any other animals unless directed by a veterinarian.

Where to Buy Fish Mox (Amoxicillin) Without a Prescription

One of the conveniences of Fish Mox is that in the United States it’s legal to purchase for aquarium use without a prescription​. This makes it easy for hobbyists to get antibiotics for their fish quickly when needed. TheFishAntibiotics.com is a trusted source that offers Fish Mox and other fish medications online with fast shipping.

• Fish Mox 250 mg (Amoxicillin), Fish Mox 500 mg (Amoxicillin) – available from TheFishAntibiotics.com without a prescription. You can find it here: 250 mg Fish Mox – Amoxicillin for Fish (No Prescription Required). This site specializes in fish antibiotics, meaning you can legally and safely order Fish Mox and have it shipped quickly to your home. The convenience of online ordering and fast shipping is a big plus, especially if you don’t have a local fish store that carries antibiotics. The product is the same amoxicillin used by many hobbyists, packaged for aquarium use. (Always ensure you’re buying from reputable vendors like this to get genuine medication.)

• No Prescription Needed: In the U.S., Fish Mox is categorized for ornamental fish use, so you do not need a veterinarian’s prescription to buy or use it​. This is perfectly legal and common – it’s sold in pet stores and online for aquarium hobby use. However, it should only be used for fish and aquatic pets. Even though it’s the same antibiotic as the human version, the manufacturing standards might differ, and self-medicating humans or other animals with fish antibiotics is not advised. Stick to using it for your fishy friends, as intended.

• Fast Shipping & Convenience: TheFishAntibiotics.com offers quick shipping options so you can start treatment as soon as possible. When a fish is sick, time is of the essence – an infection can worsen quickly. Having a reliable source to overnight or 2-day ship Fish Mox can be a lifesaver (literally for the fish). The site is also convenient because you can order other related meds or supplies at the same time.

When buying Fish Mox, you’ll usually see two versions:

• Fish Mox – the 250 mg capsules (often sold in bottles of 30, 60, or 100 capsules).

• Fish Mox Forte – the 500 mg capsules.

Choose based on your tank needs (for big tanks, the Forte 500 mg might be more convenient; for small tanks, the 250 mg is easier to measure out). If unsure, the 250 mg is a versatile choice and you can always use multiple capsules for larger volumes.

Always check the expiration date when you receive the product, and store the capsules in a cool, dry place. If stored properly, they should remain effective until the expiration date on the bottle.

Other Fish Antibiotics and Alternatives

Sometimes, amoxicillin (Fish Mox) may not be the best antibiotic for a particular infection, or a pathogen might be resistant to it. In such cases, there are other fish antibiotics available (many also sold on TheFishAntibiotics.com) that you can consider. Each antibiotic has a different spectrum of bacteria it treats. Here’s a quick look at alternatives:

• Fish Cephalexin – Cephalexin (branded as Fish Flex in some products) is another broad-spectrum antibiotic, particularly good for gram-positive bacteria. It can treat fin rot, tail rot, and some skin infections similarly to amoxicillin. Use it if amoxicillin isn’t yielding results or if the infection is known to respond better to cephalosporins.

• Fish Penicillin – Penicillin-based fish antibiotics (e.g., Fish Pen) target gram-positive bacteria primarily. They are less commonly used than amoxicillin in aquariums but can be effective for specific infections like Streptococcus-type infections in fish.

• Fish Ciprofloxacin – Ciprofloxacin (Fish Flox) is a fluoroquinolone antibiotic. It’s very potent against gram-negative bacteria and is sometimes used for serious infections like columnaris or septicemia when other meds fail. It’s a good alternative if amoxicillin isn’t working, but use with caution and follow dosing precisely, as it’s strong.

• Fish Metronidazole – Metronidazole (Fish Zole) is an antibiotic and antiprotozoal. It’s often used for internal infections, protozoan parasites, or anaerobic bacterial infections. It won’t overlap much with amoxicillin’s uses, but if a fish has internal swelling or suspected gut infection (or Hexamita in cichlids), metronidazole is a go-to.

• Fish Clindamycin – Clindamycin is an antibiotic effective against certain gram-positive bacteria and some anaerobes. It might be chosen for infections like fish wound infections or abscesses that don’t respond to other meds.

• Fish Azithromycin – Azithromycin (sometimes sold as Bird Zithro for birds, but also used for fish) is a macrolide antibiotic. It’s useful against a range of bacteria and is known for treating difficult cases of bacterial infections in corals and fish. As an alternative for fish, it can be used for infections where amoxicillin or others aren’t suitable.

• Fish Doxycycline – Doxycycline (a tetracycline antibiotic) can treat gram-positive and gram-negative infections. It’s good for ailments like fin rot, mouth rot, and some internal infections. It can be an alternative especially if a broad approach is needed and amoxicillin didn’t help.

• Fish Sulfamethoxazole – Often combined with Trimethoprim (as SMZ-TMP), this sulfa antibiotic is effective for a broad range of bacterial infections and sometimes used for resistant strains. It’s useful for fin rot, columnaris, and even some parasites (because of the trimethoprim combo).

• Fish Fluconazole – Fluconazole is actually an antifungal, not an antibiotic. It’s used to treat fungal infections in fish (like true fungus on fish bodies or fungal outbreaks on aquarium decor). While it’s not an “antibiotic,” it’s worth mentioning as another treatment available on the site, useful if you’re dealing with fungus rather than bacteria.

Each of these alternatives has its own dosing guidelines and target uses. If Fish Mox does not solve the issue, identify the infection type and consider one of the above. TheFishAntibiotics.com provides these medications without prescription as well, making it easier to obtain the right treatment. Always research the specific dosing and usage instructions for any alternative antibiotic, as they can differ from amoxicillin’s instructions. When using any antibiotic, remember to remove carbon, monitor water quality, and follow a full course of treatment to prevent resistant bacteria and ensure your fish recover fully.

Conclusion

Treating your aquarium fish with antibiotics like Fish Mox (amoxicillin) can be straightforward and effective when done correctly. The keys to success are accurate dosing by gallon, consistent daily treatment for the recommended duration, maintaining water quality (with partial water changes and no carbon filtration during treatment), and targeting the right illness with the right medication. Amoxicillin is a versatile antibiotic for fish, helping cure fin rot, columnaris, popeye, mouth fungus, and other bacterial infections that commonly trouble aquarium inhabitants.

By following the dose per gallon guidelines (typically 10–20 mg/gal, or one 250 mg capsule per 10 gallons as a convenient measure​), you can safely medicate your tank. Use the dosing chart for quick reference on common tank sizes, and don’t hesitate to open capsules and dissolve them for precision. Re-dose every 24 hours for 5-10 days​, and you’ll give your fish the best chance at a full recovery. Remember to observe your fish closely – they should start showing signs of improvement after a few days of treatment.

Fish Mox being available without a prescription means you can get it when you need it, from reputable sources like TheFishAntibiotics.com (with fast shipping and quality products). This accessibility, however, comes with a responsibility: use antibiotics wisely. Only treat bacterial infections that truly require it, and always complete the course. Misuse or overuse of antibiotics can lead to resistant strains of bacteria, which is bad news for everyone (fish included).

With proper use, amoxicillin can be a fish life-saver in your aquarium. Whether you’re a beginner who just noticed your betta’s fins rotting, or an experienced aquarist dealing with a stubborn columnaris outbreak, the information in this guide should help you dose Fish Mox confidently and effectively. Keeping a healthy tank sometimes means playing fish doctor – and now you have the prescription for success (no vet needed). Good luck with your treatment, and may your fish recover swiftly and stay healthy!

References:

1. Grange Co-op (n.d.). Fin Mox Amoxicillin 500mg – How to Use – Dosing per aquarium size and no prescription required​grangecoop.com​grangecoop.com.

2. ReefGoods (n.d.). Amoxicillin 500mg – Directions – Dissolving capsules and distribution in aquarium​