Fish Amoxicillin: Uses, Dosage, and Safe Aquarium Use
Reviewed by the Fine PetHealth Editorial Team. Written from current product labeling and manufacturer specifications. Last updated: July 2026.
Fish amoxicillin is amoxicillin prepared for ornamental aquarium fish. Aquarists use it as an antibacterial treatment for common fish infections such as fin and tail rot, dropsy, red streaks, skin ulcers, and gill disease. Today it is sold as Fix Mox, the current version of the older Fish Mox and Aqua Mox products, in 250 mg and 500 mg capsules. This guide covers what fish amoxicillin treats, how to dose it by tank size, how to use it safely, and where to buy it.
What is fish amoxicillin?
Fish amoxicillin is amoxicillin trihydrate packaged for ornamental aquarium fish and non-food pet birds. Amoxicillin is a broad-spectrum antibacterial that works against many of the bacteria behind fish infections. It is sold in pull-apart capsules and dissolved into tank water for treatment. It is made and labeled for ornamental fish only, not for fish raised for food.
Fish amoxicillin, Fish Mox, Fix Mox and Aqua Mox: are they the same?
Yes, they share the same active ingredient. Fish Mox and Fish Mox Forte (from the discontinued Thomas Labs line) and Aqua Mox were earlier amoxicillin products for aquarium use. Fix Mox from Aqua Soma Labs is the current replacement, with the same amoxicillin in 250 mg and 500 mg capsules. Fish Mox Forte was the 500 mg strength, and the 250 mg option is the standard strength. For a wider look at what replaced the old Thomas Labs range, see our guide to Thomas Labs alternatives.
What does fish amoxicillin treat?
Fish amoxicillin treats bacterial infections in ornamental fish. Common uses include fin and tail rot, dropsy (body swelling), red streaks, skin ulcers, pop-eye, hemorrhagic septicemia, and some gill infections, in both freshwater and saltwater tanks.
It works on bacteria only. It does nothing for fungal, viral, or parasitic problems such as ich, flukes, or fungus, which need different treatments. If you are not sure whether an infection is bacterial, an aquatic veterinarian or an experienced aquarist can help you confirm before you treat.
Fish amoxicillin dosage by aquarium size
The common aquarium guide for fish amoxicillin is 250 mg per 10 gallons (38 L) or 500 mg per 20 gallons (76 L) of tank water, once every 24 hours for 5 to 10 days, with a partial water change between doses. Dose by the water volume of your treatment tank, not by the number of fish. The bottle label or your aquatic veterinarian always takes priority over a general chart.
| Tank volume | Amoxicillin per dose | 250 mg capsules | 500 mg capsules |
|---|---|---|---|
| 10 gallons (38 L) | 250 mg | 1 | n/a |
| 20 gallons (76 L) | 500 mg | 2 | 1 |
| 30 gallons (114 L) | 750 mg | 3 | 1 + one 250 mg |
| 40 gallons (151 L) | 1000 mg | 4 | 2 |
| 55 gallons (208 L) | about 1375 mg | 5 to 6 | 3 |
Fish amoxicillin 500mg dosage
For the 500 mg strength (the fish amoxicillin 500mg option, equivalent to Fish Mox Forte), dissolve one 500 mg capsule per 20 gallons (76 L) of tank water, once every 24 hours for 5 to 10 days, with a partial water change between doses. Open the capsule, stir the powder into a cup of tank water, then pour it evenly around the treatment tank.
Fish amoxicillin 250mg
The 250 mg strength treats 10 gallons (38 L) per capsule on the same schedule. Use it for smaller tanks, or combine strengths to match your exact tank volume (for example, one 500 mg plus one 250 mg capsule for a 30 gallon tank).
How long should you treat?
Most fish treatments run 5 to 10 days. Finish the full course even if the fish look better early, or the infection can come back. If there is no improvement after about 5 days, or the fish get worse, stop treatment, improve water quality and aeration, and ask an aquatic veterinarian.
How to use fish amoxicillin in your aquarium safely
When you use amoxicillin for fish tanks, treat sick fish in a separate hospital or quarantine tank rather than in the main display, so you do not affect healthy tank mates. A simple, safe routine looks like this:
- Move sick fish to a dedicated hospital or quarantine tank before dosing.
- Remove activated carbon and switch off UV sterilizers, since both strip amoxicillin from the water.
- Do a 20 to 25 percent water change before the first dose to start on cleaner water.
- Open the capsule, dissolve the powder in a cup of tank water, then spread it around the tank.
- Repeat every 24 hours for 5 to 10 days, with a partial water change between doses.
- Monitor ammonia, nitrite, and temperature, and keep aeration strong throughout.
- After treatment, run a partial water change and return activated carbon to clear any leftover amoxicillin.
Do not treat display tanks that hold shrimp, snails, corals, or other invertebrates. Amoxicillin can harm them and can knock back the beneficial bacteria in your biological filter. Treat affected fish in a separate tank instead.
Is fish amoxicillin the same as amoxicillin clavulanate?
No. Fish amoxicillin is plain amoxicillin (amoxicillin trihydrate). Amoxicillin clavulanate is a different combination that adds clavulanic acid. Aquarium products such as Fix Mox contain amoxicillin only, not the clavulanate combination, so follow amoxicillin guidance rather than clavulanate guidance.
Is fish amoxicillin safe for humans?
No. Fish amoxicillin is made and labeled only for ornamental aquarium fish and non-food pet birds. It is not intended for people and should never be taken by humans. Products for ornamental animals are not held to the same standards as human care, so taking them yourself can be unsafe. The U.S. Food and Drug Administration warns against using ornamental fish products for human health. If you or someone else needs care, contact a licensed healthcare provider.
Can you use fish amoxicillin for dogs or cats?
No. Fish amoxicillin is labeled for ornamental aquarium fish and non-food pet birds only, not for dogs, cats, or other mammals. A dog or cat that needs amoxicillin should be seen by a veterinarian, who can recommend a species-appropriate product and the correct amount for the animal's weight.
Where to buy fish amoxicillin (250 mg and 500 mg)
You can buy fish amoxicillin online. Fine Pet Health sells Fix Mox, the current fish amoxicillin, in 250 mg and 500 mg strengths and in bottles of 30 or 100 capsules, shipped from a United States warehouse with clear labels and current expiration dates. Some aquarium keepers look for fish amoxicillin at general retailers, but availability there changes often, so buying from a dedicated ornamental fish supplier helps you get the strength and count you need.
If amoxicillin is not the right fit for the infection, other aquarium antibacterials target different bacteria. You can compare options in the Fish & Bird Antibacterials collection, including Fix Doxy (doxycycline) and Fix Flox (ciprofloxacin).
Frequently asked questions
Is fish amoxicillin safe for humans?
No. It is labeled only for ornamental aquarium fish and non-food pet birds and should never be taken by people. The FDA advises against using ornamental fish products for human health. See a licensed healthcare provider if you need care.
Is Fish Mox the same as human amoxicillin?
The active ingredient, amoxicillin trihydrate, is the same, but the products are not interchangeable. Fish Mox and Fix Mox are made and labeled for ornamental aquarium fish, not for people, and are not held to human care standards.
Is Fish Mox the same as amoxicillin?
Yes. Fish Mox and the current Fix Mox contain amoxicillin trihydrate, at 250 mg or 500 mg per capsule, prepared for ornamental aquarium fish.
Is fish amoxicillin the same as amoxicillin clavulanate?
No. Fish amoxicillin is plain amoxicillin. Amoxicillin clavulanate adds clavulanic acid and is a different formulation. Aquarium products like Fix Mox are amoxicillin only.
What is the fish amoxicillin 500mg dosage?
The general guide is one 500 mg capsule per 20 gallons (76 L) of tank water, once every 24 hours for 5 to 10 days, with a partial water change between doses. Measure your treatment tank volume first, and follow the bottle label.
Can you use fish amoxicillin for dogs or cats?
No. It is for ornamental aquarium fish and non-food pet birds only. A dog or cat that needs amoxicillin should be seen by a veterinarian for a species-appropriate product and amount.
Can you still buy fish amoxicillin?
Yes. The older Thomas Labs Fish Mox and Aqua Mox were discontinued, but the current fish amoxicillin, Fix Mox, is available online in 250 mg and 500 mg strengths.
Sources and further reading
- U.S. Food and Drug Administration, Ornamental Fish Drugs and You.
This article is for general educational purposes only and is not a substitute for advice from your veterinarian. Fish amoxicillin (Fix Mox) is for ornamental aquarium fish and non-food pet birds only, and not for animals raised for food or for human use. This page was prepared by the Fine PetHealth Editorial Team with AI assistance, verified against the current product label, and reviewed by a person before publishing.