AquariumStocking

How to Treat Sick Fish

Published October 3, 2026. Built by Jeremy Panasuk.

Quick answer: Test the water before you medicate. The Merck Veterinary Manual says use of prophylactic medication in the absence of diagnostic testing is strongly discouraged. Lethargy and loss of appetite in a tank only a few weeks old are the picture of new tank syndrome, which is a water problem. This page does not list drug doses.

Water first, then a diagnosis

Poor water quality is, in the Merck environmental-diseases chapter, the most common cause of environmentally induced disease. Inexpensive test kits are easy to use and reasonably accurate. The same chapter says many pet stores have limited testing and may not interpret results well, so a keeper who can test at home is in a better position than one who is handed a bottle.

The management chapter's description of new tank syndrome is the illness people often try to medicate: usually within the first 6 weeks, fish become lethargic, anorectic, and frequently die, and ammonia, nitrite, or both are often high enough to be toxic. Treatment named there is less food, water changes, chloride for nitrite toxicity, and a look at the biofilter. It is not a shop medicine. The clocks and the separate ammonia thresholds are written out on how to fix cloudy water in a new tank. They are not repeated here as one blended number.

What to hold onto from the environmental chapter, without collapsing its thresholds: freshwater fish can usually tolerate total ammonia nitrogen under 1 mg/L; a home-aquarium TAN of 1.0 is not a huge concern unless pH exceeds 8.5, and the un-ionized fraction should still be calculated; un-ionized ammonia under 0.05 mg/L is not considered harmful, with species and situation differences; a calculated 0.05 mg/L already causes gill changes and is considered harmful; 2 mg/L un-ionized ammonia is lethal for many fish. A table row for ammonia toxicity (new tank syndrome) is a further line: un-ionized ammonia above 1 mg/L and pH above 8. Signs of ammonia exposure include lethargy and poor appetite. Acute toxicity can look like spinning, disorientation, and convulsions. Nitrite toxicity sends fish to the surface, piping, and a water change can correct it when the tank is small enough that a change is practical. The chapter says a change may not be practical for large ponds.

If un-ionized ammonia is high, that chapter says a water change of at least 50% should be done if possible, after you check whether the tap water contains ammonia or chloramines. Feeding should be discontinued or substantially reduced until the problem is corrected. Those are water actions. They are not a prescription.

Why this page has no doses

The management chapter discusses many antimicrobials and parasiticides, and it says use of prophylactic medication without diagnostic testing is strongly discouraged and may contribute to resistant bacterial infection and other complications. Copying a dose from that chapter onto a fish you have not diagnosed would be the practice the sentence discourages. Manufacturer labels are the other place a dose could be cited. This page does not quote one, because no specific product is being recommended. An aquatic veterinarian chooses the drug after an examination, a water test, and, when it will change the treatment, a skin scrape, gill exam, or culture.

Dips in the same chapter depend on the species. For freshwater or marine fish, the pH and temperature of the dip water should be the same as the tank water, and a fish that rolls onto its side comes out immediately. A veterinarian chooses whether a dip is appropriate. This page does not give the concentration or the contact time.

Worked example: the tank can be the illness

Before you conclude a fish needs medicine, run the load through this site's calculator. Points are a planning estimate for a cycled freshwater tank, not a diagnosis.

10 gallons, normal setup, 12 small fish: 12 points, capacity 10, 120%, overstocked.

Points are 12 × 1 = 12. Capacity is round(10 × 1.0) = 10. Percent is round(12 / 10 × 100) = 120. Status is overstocked because 120 is above 100. The environmental chapter says beginning aquarists overstock and overfeed, and then ammonia or nitrite spikes. A fish gasping in that tank may be in brown blood or an ammonia event. Medicine on top of 120% does not add biofilter. Enter the same inputs on the stocking calculator. If your count is lower and the fish are still sick, the calculator has done its job by ruling the load in or out. It has not named a disease.

Heater and filter on that 10-gallon tank, so a quarantine or hospital tank is at least the right temperature and flow while you wait for a vet: room 68°F to 78°F is a 10°F rise, 4 watts per gallon, 40 watts raw, next standard size 50 watts. Normal filter turnover is 5 times, ideal 50 gallons per hour, derated rating 60 gallons per hour. The management chapter's home quarantine tank is a 10-gallon with a sponge filter, a small air pump, and a heater. Use that shape if the sick fish have to leave the display tank. How long they stay apart is the 30-day minimum on how to add fish.

A common mistake

Buying a broad medication because a fish looks off, without an ammonia and nitrite test. That is the prophylactic use the manual discourages. The second mistake is treating a 120% load as a parasite problem. Check the points first.

Limits, and when to call a veterinarian

Call an aquatic veterinarian when fish are dying, when they spin or convulse, when several fish are affected, or when a water test is already in range and the fish are still sick. This page cannot see the fish. If a fish has already died and you want a diagnosis, the management chapter says a fish dead less than 24 hours and kept at 4°C has diagnostic value, and a fish placed in a freezer has limited diagnostic value. Fish decompose quickly. That storage note is for a necropsy, not a home freezer treatment. Euthanasia, if a veterinarian recommends it, is a separate decision with its own wording on how to humanely euthanize a fish.

For the trip, the same chapter says a live animal can be transported in a cooler with a battery-powered aerator, or in a sturdy plastic bag with just enough water to cover the fish. A separate water sample, with no air bubbles, is transported on ice, and a minimum of 500 mL should be requested. Bring the water. The vet cannot interpret a fish without it.

Shop an aquarium liquid test kit on Amazon

Affiliate Affiliate disclosure: as an Amazon Associate, we earn from qualifying purchases — at no cost to you. A test kit is listed because the water has to be tested before any medicine. No medication is linked from this page.

Compiled with AI assistance from the sources below. Spot an error? Email jpanasuk@gmail.com.

Sources