Fish Mox: What It Is and How to Use It Safely

September 10, 2026

What Is Fish Mox?

Fish Mox is the common name for Amoxicillin formulated for aquarium use. It's one of the most trusted antibiotics among fish hobbyists and experienced aquarists for treating a wide range of bacterial infections in freshwater and saltwater fish.

If you've noticed your fish looking lethargic, developing sores, losing scales, or showing signs of fin rot — bacterial infection could be the culprit, and Fish Mox is often the go-to solution.

What Does Fish Mox Treat?

Fish Mox is effective against a broad spectrum of gram-positive and some gram-negative bacteria. Common conditions it's used to treat include:

Fin rot — fraying or disintegrating fins caused by bacterial infection
Columnaris — white or gray patches on the body or mouth area
Dropsy — bloating caused by internal bacterial infection
Popeye — swollen, protruding eyes due to bacterial buildup behind the eye
Open wounds and ulcers — secondary infections from injury or parasite damage

How to Use Fish Mox

Fish Mox comes in capsule form (typically 250mg or 500mg). Here's the general treatment protocol most aquarists follow:

Remove activated carbon from your filter before treatment — carbon absorbs medication and renders it ineffective.
Dose correctly — the standard dose is one 250mg capsule per 10 gallons of water, or one 500mg capsule per 20 gallons. Always follow the product label.
Treat every 24 hours for 10 days, performing a 25% water change before each new dose.
Isolate sick fish in a hospital tank when possible to avoid disrupting your main tank's beneficial bacteria.
Complete the full course — stopping early can allow bacteria to survive and become resistant.

Important Tips

Fish Mox will cloud your water slightly — this is normal and will clear after treatment ends.
Do not use in tanks with invertebrates (shrimp, snails, crabs) as antibiotics can harm them.
Always diagnose correctly before treating — not all fish illness is bacterial. Parasitic and fungal conditions require different treatments.
Store Fish Mox in a cool, dry place away from direct sunlight.