Author: PetRemedyUSA

Typical label directions for Fish Mox are one capsule per 20 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 20 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.

Quick dosing math by tank size

At the typical label rate of one 250 mg capsule per 20 gallons of water, a single dose looks like this. The 500 mg Forte strength needs half as many capsules for the same total milligrams.

Tank size	250 mg capsules per dose	500 mg Forte capsules per dose
10 gallons	½	¼ (the 250 mg strength is easier here)
20 gallons	1	½
40 gallons	2	1
55 gallons	2½–3	1½
75 gallons	3½–4	2
125 gallons	6–6½	3–3½

Capsule contents can be divided for half doses. Always follow the directions printed on your bottle — they take priority over any chart.

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.

Fish Mox and Fish Mox Forte are the same medication — amoxicillin for ornamental fish — at two strengths: 250 mg and 500 mg per capsule. The right choice comes down to tank volume: the 250 mg strength is the standard pick for most home aquariums, while the 500 mg forte strength saves capsules on large tanks and multi-tank systems.

Side-by-side comparison

	Fish Mox 250 mg	Fish Mox Forte 500 mg
Amoxicillin per capsule	250 mg	500 mg
Capsules per bottle	100	100
Typical fit	Tanks up to ~55 gallons	Large tanks, ponds’ quarantine systems, fish rooms
Capsules per treatment day*	More on big tanks	Half as many for the same total dose
Also searched as	fishmox, fish-mox	fish mox forte, fishmox forte

*Always follow the directions printed on your bottle — strengths and directions vary by label.

When the 250 mg strength makes sense

If you keep a single community tank, a betta setup, or anything up to roughly a 55-gallon aquarium, the 250 mg strength is the straightforward choice. Dosing granularity is actually an advantage in smaller volumes of water — it’s easier to match the label’s per-gallon directions without splitting capsules.

When to step up to Forte 500 mg

Keepers dosing 75–125+ gallon tanks, multiple quarantine systems, or a whole fish room burn through capsules quickly at 250 mg. The forte strength delivers the same total amoxicillin with half the capsules, which usually works out cheaper per treatment course.

Everything else is identical

Same active ingredient, same factory-sealed bottle format, same use cases — fin rot, mouth rot, body sores and other common bacterial conditions in ornamental fish. Both ship from our warehouse in 1–2 business days. See the full Fish Mox overview for what it treats and how it’s dosed, or our plain-English explainer if you’re new to fish antibiotics.

Fish Mox is a marketplace name used for amoxicillin products marketed for ornamental aquarium fish. Amoxicillin is a penicillin-class antibiotic. The words “Fish Mox” describe how a product is marketed; the exact strength, count, and directions come from the label on the bottle you receive.

What does the Fish Mox label mean?

A listing should identify the final product, active ingredient, strength, count, intended species, directions, warnings, and responsible company. Check the label on the item you receive rather than relying only on a product name or listing image.

Every bottle should state its active ingredient, strength per capsule, capsule count, intended species, and directions. Those printed details — not the product name — tell you exactly what you have and how to use it. PetRemedyUSA lists the strength and count of every item on its product page so you can match the bottle to your order.

Fish Mox 250 mg and Fish Mox 500 mg

Fish Mox-style listings commonly distinguish products by the amount of amoxicillin stated per capsule. PetRemedyUSA currently lists Fish Mox 250 mg and Fish Mox 500 mg. Review each current listing and the received label for the exact strength and count.

When to talk to a veterinarian

PetRemedyUSA does not diagnose fish diseases or provide individualized treatment or dosing instructions. A licensed veterinarian who treats aquatic animals can evaluate the fish, water conditions, diagnosis, and legally appropriate treatment options.

Can people take Fish Mox?

No. Never use an animal-labeled product as a substitute for medicine prescribed for a person. Human medication should come from a licensed healthcare professional and a legitimate human-drug source.

What should buyers verify?

• The item, active ingredient, strength, count, and intended species match the order.
• The packaging and label are present, readable, and undamaged.
• Any approval, conditional-approval, or index claim is supported by the application or MIF information required on the label.
• The item is not used for people or animals intended for food.

If the received item differs from the order or its label is missing or unreadable, do not use it and contact customer support.

Related Fish Mox guides

• Fish Mox overview — both strengths in stock
• How to dose Fish Mox: label directions, hospital tanks & filters
• Fish Mox vs Fish Mox Forte: which strength fits your tank
• How to treat fin rot in aquarium fish
• How to set up a hospital (quarantine) tank

A hospital tank is a small, bare-bones aquarium used to treat sick fish away from your display tank — and it is the single best investment a fish keeper can make. Treating in a separate tank protects your main tank’s beneficial bacteria from antibiotics, lets you dose medication such as Fish Mox (amoxicillin) accurately in a known volume of water, and removes the stress of competition while a fish recovers. A basic 10-gallon setup costs little and takes under an hour to prepare.

What do you need for a hospital tank?

• A 10–20 gallon tank (bigger for large cichlids or goldfish)
• A simple sponge filter — gentle flow, no carbon, and it can be pre-seeded in your main tank’s filter so it arrives already cycled
• A heater and thermometer matched to your display tank’s temperature
• Bare bottom, no substrate — easier to clean and lets you see waste and uneaten food
• A hiding place such as a PVC elbow or smooth clay pot to reduce stress
• A dedicated net and bucket so you don’t carry pathogens back to the display tank

How do you set it up quickly when a fish is already sick?

1. Fill the tank with water from your display tank (a big water change on the main tank at the same time is a bonus), topping up with dechlorinated fresh water.
2. Move a seeded sponge filter or a handful of filter media from the main tank into the hospital filter — this gives you instant biological filtration.
3. Match the temperature before transferring the fish, then acclimate it gently.
4. Keep lighting dim and the tank partially covered — sick fish settle faster in low light.

How do you medicate in a hospital tank?

Dose for the hospital tank’s actual water volume, following the medication label — for most aquarium antibiotics in our fish range, dose rates vary by product — from one capsule per 10 gallons to one tablet per 40 gallons every 24 hours, depending on the medication, with a partial water change between doses. Because there’s no carbon and a known volume, dosing is far more precise than in a decorated display tank. Watch ammonia daily with a test kit: antibiotics can slow even a seeded filter, and a 25% daily water change (re-dosing medication proportionally per the label) keeps the water safe. After treatment ends, run carbon for a day or two to pull out residual medication before returning the fish.

Should new fish be quarantined too?

Yes — the same tank doubles as a quarantine tank. Two to four weeks of observation for every new arrival catches most infections, parasites, and fin rot before they ever reach your display tank. Many experienced keepers consider quarantine non-negotiable; it is far easier to treat one new fish in a bare 10-gallon than an entire community tank.

Fin rot is a bacterial infection that eats away at a fish’s fins, leaving them ragged, frayed, or discolored at the edges. It is one of the most common diseases in home aquariums, and it is very treatable: improve the water quality that allowed the bacteria to take hold, then treat the infection itself — typically with a broad-spectrum aquarium antibiotic such as amoxicillin. Caught early, most fish regrow their fins completely.

What does fin rot look like?

• Fin edges that look ragged, torn, or “melted” rather than smooth
• White, black, or brown discoloration along the fin margins
• Fins that get progressively shorter over days or weeks
• In advanced cases, redness or inflammation at the fin base — a sign the infection is reaching the body

Fin rot is usually caused by common aquarium bacteria that attack fish whose defenses are lowered by stress — poor water quality, overcrowding, aggressive tank mates nipping fins, or sudden temperature swings.

How do you treat fin rot?

1. Test and fix the water first. Check ammonia, nitrite, and nitrate. Do a 25–50% water change, vacuum the substrate, and remove any decaying plant matter or uneaten food. Many mild cases start improving from this step alone.
2. Move the fish to a hospital tank if you can. Treating in a separate tank protects your main tank’s biological filter and lets you dose accurately. (See our hospital tank guide.)
3. Remove activated carbon from the filter — carbon absorbs medication and stops it working.
4. Treat with a broad-spectrum aquarium antibiotic. Fish keepers commonly use amoxicillin (such as Fish Mox 250mg) for fin and tail rot — both strengths and dosing basics are covered on our Fish Mox page. Typical label directions dose one capsule per 20 gallons every 24 hours with partial water changes between doses — always follow your bottle’s label. For stubborn cases, keepers sometimes step up to combination sulfa treatments like Fish Sulfa.
5. Complete the full course — usually 5–10 days per the label. Stopping early lets the infection rebound.
6. Finish with a water change and fresh carbon to clear residual medication.

How long does fin rot take to heal?

With clean water and proper treatment, you should see the fin edges stop receding within about a week, and regrowth — often clear or pale at first — over the following two to four weeks. Severe cases that reached the fin base take longer and may leave some permanent shortening. If a fish shows no improvement after a full treatment course, reassess the water, confirm the diagnosis (fungus and nipping damage look similar), and consult a fish-experienced veterinarian.

How do you prevent fin rot?

• Keep ammonia and nitrite at 0 ppm with regular water changes
• Avoid overstocking and fin-nipping tank mate combinations
• Quarantine new fish for 2–4 weeks before adding them to the display tank
• Feed quality food in amounts the fish finish within a couple of minutes