Medication dosing for common fish diseases

Most fish medications are dosed per volume: "add X per 10 gallons." The problem is that the box means US gallons, the volume printed on your tank is the total capacity (not the water volume after substrate, rocks, and equipment displace some of it), and some medications are toxic to scaleless fish, invertebrates, or plants at the standard dose. The medication dosing calculator on this site adjusts for actual water volume after displacement and flags scaleless species, but the identification and treatment decisions are covered here.

Quarantine first

Every treatment works better in a quarantine tank than in the display tank. A 40L tank with a sponge filter (seeded from the display tank's filter), a heater, and no substrate is the standard quarantine setup. Medications stress the biological filter; running them in a bare quarantine tank means the display tank's cycle stays intact.

If quarantine isn't possible (the entire display tank is infected, or the quarantine tank isn't cycled), treat the display tank but monitor ammonia and nitrite during and after treatment. Some medications (antibiotics especially) kill beneficial bacteria alongside the target pathogen.

Ich (white spot disease)

Pathogen: Ichthyophthirius multifiliis, a protozoan parasite.

Identification: Small white dots, 0.5-1 mm, scattered across the body and fins. Looks like grains of salt. Fish may flash (rub against surfaces) before spots appear. Severe infections show dozens to hundreds of spots and rapid gill movement.

Life cycle matters for treatment. The visible white spot is the feeding stage (trophont), embedded in the skin. It's protected from medication at this stage. When it matures, it drops off the fish, falls to the substrate, and encysts. Inside the cyst, it divides into hundreds of free-swimming theronts. The theronts are the vulnerable stage; medication kills them in the water column before they attach to a fish.

Treatment: Raise temperature to 28-30°C. This accelerates the life cycle so the encysted stage releases theronts faster, shortening the treatment window. Dose the tank with a malachite green / formalin combination (brand names: Ich-X by Hikari, Super Ich Cure by API). Follow the product dosage per actual water volume. Treat every 48 hours for 7-10 days to cover multiple life cycles.

Scaleless fish and invertebrates: Malachite green is toxic to scaleless species (loaches, catfish, elephant nose, stingrays) and lethal to shrimp and snails at full dose. For scaleless fish, dose at half the standard rate. For shrimp and snail tanks, raise temperature alone (30°C) and add aquarium salt at 1 tablespoon per 20 liters. Salt kills the free-swimming theronts but is tolerated by most fish. Shrimp tolerate low salt concentrations; snails less so. Remove carbon from the filter during treatment; it absorbs the medication.

Fin rot

Pathogen: Usually Aeromonas or Pseudomonas bacteria. Opportunistic; attacks fish already stressed by poor water quality, aggression, or injury.

Identification: Ragged, fraying fin edges. May progress to fin tissue dissolving, exposing the fin rays. In severe cases, the rot reaches the fin base and enters the body, which is usually fatal. Sometimes a white or milky edge precedes visible tissue loss.

Treatment: Start by fixing the underlying cause. In most cases, fin rot resolves with clean water alone. Large water changes (50% every other day for a week), good filtration, and removing the source of stress (aggressive tankmate, overcrowding, ammonia spike) are the first line.

If the rot is progressing despite clean water, dose with an antibiotic. Kanaplex (kanamycin) or Maracyn 2 (minocycline) are effective against the gram-negative bacteria that cause most fin rot. For mild cases, Melafix (tea tree oil extract) or aquarium salt (1 tbsp per 20L for 7-10 days) may be sufficient, but neither is a real antibiotic and they won't stop a serious bacterial infection.

Columnaris

Pathogen: Flavobacterium columnare. A gram-negative bacterium. Not a fungus despite the fuzzy appearance.

Identification: White or greyish patches on the body, mouth, or gills. Can look cottony (hence the "mouth fungus" nickname, which is misleading). Mouth sores that erode the lip tissue. Saddleback lesion (a pale band behind the dorsal fin). Rapid gill decay in the gill-attack form, which kills fast (12-48 hours). Progresses much faster than fin rot.

Treatment: Kanaplex (kanamycin) in combination with Furan-2 (nitrofurazone). This combination targets gram-negative bacteria from two angles. Dose per actual water volume. Lower the temperature to 24°C if the fish tolerate it; Flavobacterium columnare grows faster in warm water (above 28°C outbreaks are more virulent). Treatment duration: 7-10 days.

Columnaris is contagious and fast. Quarantine and treat immediately. Waiting "to see if it gets worse" usually means losing the fish.

Velvet

Pathogen: Piscinoodinium pillulare (freshwater) or Amyloodinium ocellatum (brackish/marine). A dinoflagellate parasite.

Identification: Fine gold or rust-colored dust on the body, much finer than ich spots. Easier to see with a flashlight angled across the fish in a dark room. Fish clamp fins, flash against objects, and breathe rapidly. Lethal if untreated; velvet kills faster than ich because it attacks the gills heavily.

Treatment: Dim the lights or black out the tank (the parasite photosynthesizes). Dose with copper-based medication (Seachem Cupramine or Copper Power). Copper kills the free-swimming dinospore stage. Raise temperature to 28-30°C to accelerate the life cycle, same principle as ich.

Critical warning: Copper kills invertebrates. All shrimp and snails die at therapeutic copper doses. Remove them before treatment. Copper also binds to some substrates (especially porous rock and some aquasoils) and can leach out for months afterward, making the tank permanently uninhabitable for shrimp. If treating in the display tank, assume the tank is no longer shrimp-safe.

Internal parasites

Pathogens: Various. Common ones include Hexamita (causes hole-in-the-head in cichlids), Camallanus (red worms protruding from the vent), and Spironucleus (flagellate causing bloating and white stringy feces).

Identification varies by parasite:

• Camallanus worms: Thin red threads (1-2 cm) visibly protruding from the fish's vent. Unmistakable once you see them.
• Hexamita / hole-in-the-head: Pitting erosion on the head, especially around the lateral line pores. Common in cichlids (oscars, discus, angelfish). Linked to Hexamita and/or poor nutrition (specifically vitamin and mineral deficiency).
• General internal parasites: White, stringy feces (instead of the normal dark, solid strand), bloating, wasting (fish eats but loses weight), and lethargy.

Treatment: Levamisole (for Camallanus worms specifically; it paralyzes the worm so the fish expels it), available from fish-medication suppliers or agricultural supply stores. General anti-parasitic: metronidazole (Seachem MetroPlex) dosed in food or in the water. Metronidazole is effective against Hexamita and most flagellate parasites. For hole-in-the-head, improve diet (vitamin-enriched frozen food, quality pellets) alongside metronidazole treatment.

What not to mix

Medications interact. Combining treatments without knowing the chemistry risks poisoning the fish worse than the disease.

Do not combine copper with malachite green. Both are heavy-metal-based; combined toxicity is additive and can kill fish at doses that would be safe individually.

Do not dose antibiotics and malachite green simultaneously. Malachite green suppresses the nitrifying bacteria in the filter. Adding an antibiotic that also stresses the biofilter (most do) can crash the nitrogen cycle entirely, causing an ammonia spike on top of the disease.

Do not use Melafix or Pimafix with labyrinth fish (bettas, gouramis). The tea-tree oil coats the labyrinth organ and can impair their ability to breathe atmospheric air. This claim is debated among hobbyists, but the risk is real enough that most experienced betta keepers avoid these products.

Complete the full treatment course. Cutting a medication short because the fish looks better selects for resistant organisms. The visible symptoms resolve before the pathogen is fully eliminated.

The medication dosing calculator on this site accounts for actual water volume (minus substrate and hardscape displacement), flags scaleless species at half-dose, and tracks which medications are incompatible.