How to Dose Fish Mox: Label Directions, Hospital Tanks & Filter Safety

Typical label directions for Fish Mox 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. The printed label on your bottle always takes priority — this guide covers the process around the dose: where to treat, what to remove, and how to protect your filter.

Step-by-step treatment process

1. Move the sick fish to a hospital tank if you can. A bare-bottom 10–20 gallon setup with a cycled sponge filter and a heater is ideal — our hospital tank guide shows a full setup. Treating separately protects your display tank’s biofilter, plants and invertebrates.
2. Remove activated carbon from the filter. Carbon strips medication out of the water and will quietly cancel your treatment.
3. Open the capsule and dose per the label — typically one capsule per 10 gallons, sprinkled into the tank water every 24 hours.
4. Do the partial water change the label calls for between doses, then re-dose for the new water volume according to the directions.
5. Complete the full course stated on the label even if the fish looks better early — stopping antibiotics halfway is how resistant infections happen.

Protecting your biological filter

Antibiotics can’t tell the difference between the bacteria infecting your fish and the beneficial bacteria running your nitrogen cycle. Test ammonia and nitrite daily during treatment and for a week after. If either rises, do an extra water change and cut back feeding. This is the single biggest reason to treat in a separate hospital tank.

Which strength are you dosing?

Fish Mox comes as 250 mg capsules (standard for most tanks) and 500 mg “Forte” capsules (fewer capsules for big tanks). The label directions differ by strength — check our 250 vs 500 comparison if you’re unsure which bottle fits your setup, or start at the Fish Mox overview.

When to stop or get help

If a fish shows no improvement after a full labeled course, don’t simply repeat antibiotics indefinitely — re-check water parameters (they cause most “stubborn infections”), reconsider the diagnosis (fungal and parasitic problems mimic bacterial ones), and consult an aquatic veterinarian for valuable or sentimental fish.