Category: Fish Care Guides

Aquarium antibiotics work best, and do the least harm, when they are used for the right problem and in the right way. These are the questions we hear most.

Are fish antibiotics safe for people?

No. Fish antibiotics are sold for ornamental aquarium fish only. They are not made, tested or labeled for people. If you are unwell, see a doctor.

Do I need to remove carbon from my filter?

Yes. Activated carbon absorbs many medications, so remove it during treatment. You can add fresh carbon afterwards to help clear any remaining medication.

Will antibiotics harm my filter bacteria?

Some can slow the beneficial bacteria that process ammonia. That is one reason to treat in a separate hospital tank and to test ammonia and nitrite daily during treatment.

Can I mix two medications?

Don’t combine antibiotics or other treatments unless the labels or an aquatic veterinarian say it’s safe.

Why finish the full course?

Stopping early can leave surviving bacteria that are harder to treat next time. Follow the label for the full treatment time.

How should I store and dispose of them?

Keep bottles closed, dry, at room temperature and out of reach of children and pets. Don’t pour leftover medication down the drain; use a local drug take-back program or follow your local disposal rules.

Choosing a product? See How to choose an aquarium antibiotic or browse all fish antibiotics.

Fish antibiotics are for ornamental aquarium fish only: not for human use and not for food-producing animals. This guide is general information, not veterinary advice; for valuable or persistently sick fish, ask an aquatic veterinarian. Last reviewed: November 2026.

Fin rot is one of the most common problems in home aquariums. The fins look ragged, split or shortened, often with white, black or red edges. It is usually a bacterial infection that takes hold when fish are stressed.

Common causes

• Poor water quality: ammonia, nitrite or high nitrate from overfeeding, overcrowding or too few water changes.
• Stress from sudden temperature changes, shipping or a new tank.
• Injuries from fin-nipping tankmates or sharp decorations.

Prevention

• Test the water regularly and do partial water changes on a schedule.
• Avoid overcrowding and overfeeding.
• Choose peaceful tankmates for long-finned fish such as bettas and fancy goldfish.
• Quarantine new fish before adding them to your main tank.

Treatment

1. Fix the water first. Mild fin rot often stops once the water is clean and stable, and fins slowly regrow.
2. Watch for progress. If the damage keeps spreading after the water is right, or reaches the body, an antibiotic may be needed.
3. Treat in a hospital tank with a product labeled for fin rot, such as Fix Flox, Fix Mox or Fix Flex, following the label and finishing the full course.

Fins regrow over several weeks; new growth is often clear at first. If the fish stops eating, the rot reaches the body or several fish are affected, ask an aquatic veterinarian.

Fish antibiotics are for ornamental aquarium fish only: not for human use and not for food-producing animals. This guide is general information, not veterinary advice; for valuable or persistently sick fish, ask an aquatic veterinarian. Last reviewed: November 2026.

A hospital tank is a small, separate aquarium used to treat sick fish. It keeps medication away from your display tank’s plants, invertebrates and filter bacteria, lets you dose accurately and makes it easier to watch the patient.

What you need

• A 10–20 gallon tank (larger for big fish), with a lid.
• A heater and thermometer set to the same temperature as the main tank.
• A sponge filter or air stone. Medications and warm water lower oxygen, so keep good surface movement.
• No gravel or substrate: a bare bottom is easier to keep clean.
• A simple hiding place, such as a clean pot or PVC pipe, to reduce stress.
• A water test kit for ammonia and nitrite.

Setting it up

1. Fill the tank with water from your main aquarium, or dechlorinated tap water matched for temperature and pH.
2. If you can, run the sponge filter in the main tank for a few weeks beforehand so it carries beneficial bacteria.
3. Do not use activated carbon: it removes medication from the water.
4. Move the sick fish gently, in a container rather than chasing it with a net where possible.

During treatment

• Follow the dosing and water-change directions on the medication label.
• Test ammonia and nitrite daily: medications can slow beneficial bacteria, and ammonia can build up quickly in a small tank.
• Feed lightly and remove uneaten food.
• Keep the lights dim and the area quiet.

After treatment

Return the fish only when it has finished the full course and looked healthy for several days. Then empty, clean and dry the hospital tank and equipment so it is ready for next time.

Not sure which medication to use? Read How to choose an aquarium antibiotic.

Fish antibiotics are for ornamental aquarium fish only: not for human use and not for food-producing animals. This guide is general information, not veterinary advice; for valuable or persistently sick fish, ask an aquatic veterinarian. Last reviewed: November 2026.

Choosing the right fish antibiotic starts with confirming that the problem is actually bacterial. Antibiotics do nothing against parasites, fungus or poor water, and using them unnecessarily stresses fish and the tank’s beneficial bacteria.

Step 1: Test the water first

Before any medication, test ammonia, nitrite, nitrate, pH and temperature. Ammonia or nitrite above zero, or a sudden temperature swing, can cause clamped fins, frayed fins, gasping and lethargy on their own. Fix the water with partial water changes and these problems often clear up without medication.

Step 2: Rule out parasites and fungus

• White salt-like spots on the body and fins usually mean ich, a parasite: antibiotics will not treat it.
• Fluffy, cotton-wool tufts on wounds or eggs are often true fungus.
• Flashing (rubbing against objects) points to parasites or irritation more than bacteria.

Step 3: Match the signs to the label

Most bacterial infections in aquarium fish are caused by gram-negative bacteria, while some skin and soft-tissue infections are gram-positive. The table shows which products in our range are labeled for which problems. Always read the label of the product you choose.

Step 4: Treat carefully

• Use one antibiotic at a time and follow the dosing directions on the label.
• Treat in a hospital tank where possible, and remove activated carbon from the filter.
• Finish the full course, even if the fish look better sooner.
• If there is no improvement by the end of the course, re-check the water and ask an aquatic veterinarian before trying another product.

Browse all fish antibiotics.

Fish antibiotics are for ornamental aquarium fish only: not for human use and not for food-producing animals. This guide is general information, not veterinary advice; for valuable or persistently sick fish, ask an aquatic veterinarian. Last reviewed: November 2026.