What Is Fish Mox? Amoxicillin for Aquarium Fish, Explained

Fish Mox is the common name for amoxicillin capsules packaged and sold for ornamental aquarium fish. Amoxicillin is a broad-spectrum, penicillin-class antibiotic, and aquarium hobbyists use Fish Mox to help manage common bacterial conditions such as fin and tail rot, mouth rot, and body ulcers. It is sold without a prescription for aquarium use only.

What does Fish Mox treat?

Hobbyists reach for amoxicillin when a fish shows signs of a gram-positive or mixed bacterial infection. The most common uses are:

• Fin and tail rot — ragged, frayed or discolored fin edges (see our step-by-step fin rot guide)
• Mouth rot — white, eroded tissue around the mouth
• Body sores, ulcers and red streaks on the flanks or belly
• Bacterial complications of dropsy and red pest

Like any antibiotic, it works best when started early and combined with clean water: infections almost always begin with a stressor such as poor water quality, injury or overcrowding.

Fish Mox vs Fish Mox Forte — what’s the difference?

They are the same medication at two strengths. Standard Fish Mox is 250 mg of amoxicillin per capsule; Fish Mox Forte is 500 mg. The 250 mg strength suits most home aquariums, while keepers with large tanks or multi-tank fish rooms often prefer the forte strength so fewer capsules are needed per dose. Our full Fish Mox vs Forte comparison covers when each makes sense.

How do hobbyists dose Fish Mox?

Typical label directions for this type of product are one capsule per 10 gallons of aquarium water every 24 hours, with a partial water change between doses, for the number of days stated on the label. Treat in a separate hospital tank when possible, remove activated carbon (it strips medication from the water), and keep an eye on ammonia and nitrite, since antibiotics can knock back the beneficial bacteria in your filter. Our Fish Mox dosing guide walks through the whole process.

Is Fish Mox safe for people?

No. Fish Mox is not for human use. It is packaged for ornamental fish, and it is not manufactured, tested or labeled to the standards required for human medicine. If you need an antibiotic, see a licensed physician — taking the wrong antibiotic, at the wrong dose, for the wrong infection can cause real harm.

Where can I buy Fish Mox?

Both strengths are in stock on our Fish Mox page — factory-sealed bottles, guest credit-card checkout, and tracked US shipping in 1–2 business days.