Medication dosing for common fish diseases

Ich, fin rot, columnaris, velvet, dropsy, fungal infections, and internal parasites. How to identify each, dose for your actual tank volume, set up a hospital tank, and what not to mix.

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. Follow each product's label schedule (Ich-X is dosed daily; some products are every other day with a partial water change between doses) 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.

Dropsy

Dropsy isn't a disease; it's a symptom. The fish's body swells and the scales protrude outward, giving a pinecone-like appearance when viewed from above. Fluid accumulates in the body cavity faster than the fish's kidneys can remove it.

Causes: Bacterial infection of the kidneys (most commonly Aeromonas), organ failure from chronic stress or poor water quality, viral infection, or internal tumors. By the time a fish shows the pinecone appearance, the internal damage is usually severe.

Treatment: Isolate in a hospital tank. Epsom salt (magnesium sulfate) at 1 tablespoon per 20 liters helps draw fluid from the body through osmotic action. Antibacterial food containing kanamycin (Seachem KanaPlex mixed into food using Seachem Focus as a binder) addresses bacterial infection. Feed medicated food for 7-10 days.

Prognosis: Poor. Dropsy caught very early (slight body swelling, scales not yet raised) responds to treatment maybe 30-40% of the time. Full pineconing with lethargy and loss of appetite has a survival rate below 10%. The underlying organ damage is often irreversible. Prevention (clean water, proper nutrition, low stress) is far more effective than treatment.

Fungal infections (Saprolegnia and related)

True fungal infections produce cottony, white or gray tufts on the body, fins, or gills. They look distinctly different from bacterial columnaris (which is flat and patchy rather than fluffy and tufted). Fungus attacks damaged tissue: injuries from fighting, net damage, handling, or untreated bacterial lesions become entry points.

Treatment: Methylene blue (dip treatment at 2-3 mg/L for 10-30 minutes, or add to the quarantine tank at the same concentration) is effective against most aquarium fungi. API Pimafix (contains bay oil) has mild antifungal properties and is safe for the main tank. For stubborn cases, a combination of methylene blue and salt (1-2 g/L) addresses both fungal and secondary bacterial infection.

On eggs: Fungus on fish eggs is the #1 cause of spawn failure. Methylene blue at 1-2 ppm in the breeding tank prevents fungal colonization of unfertilized eggs (which are the first to fungus, then the infection spreads to viable eggs). Remove any opaque white eggs daily; they're dead and serve as fungal nurseries.

Setting up a hospital tank

A hospital tank (quarantine tank used for treatment) doesn't need to be elaborate. A 20-40 liter container (a plastic storage bin works), a small sponge filter or air stone, a heater, and a lid. No substrate, no decorations (they absorb medication and are hard to sterilize). A piece of PVC pipe or a terracotta pot gives the fish a place to hide, which reduces stress.

Water source: Fill with water from the main tank so the parameters match. The fish is already stressed from disease; a sudden change in pH or temperature makes it worse.

Filtration: A sponge filter seeded from the main tank provides biological filtration. Note that many medications kill nitrifying bacteria, so monitor ammonia daily during treatment and do water changes as needed. Some keepers skip biological filtration entirely in a hospital tank and just do daily 50% water changes to manage ammonia, re-dosing medication after each change.

After treatment: Sterilize the hospital tank before storing it. Rinse with a bleach solution (1 part bleach to 10 parts water), soak for 30 minutes, rinse thoroughly with clean water, and air dry. The sponge filter can be discarded and replaced (they're cheap) or bleached and rinsed.

Post-treatment recovery

After a disease treatment is completed, the fish's immune system is depleted and the biofilter may be compromised. The next 2-3 weeks are a recovery period:

Water quality. Test ammonia and nitrite daily for a week after completing any antibiotic treatment. The biofilter may need to partially re-establish. Do water changes if ammonia or nitrite appear.

Feeding. Resume normal feeding gradually. Offer high-quality, vitamin-enriched food (frozen bloodworms, brine shrimp, or pellets soaked in a vitamin supplement like Vitachem). A recovering fish needs good nutrition to rebuild tissue and immune function.

Activated carbon. Run activated carbon in the filter for 48-72 hours after completing medication to remove residual drugs from the water. Replace the carbon after this period (it saturates quickly when absorbing medications).

Observation. Watch for relapse. Some diseases (ich in particular) have encysted stages that survive treatment and re-emerge days later. If symptoms return within a week of completing treatment, dose a second round at the same concentration and duration.

Medication shelf life

Liquid medications lose potency over time, especially after opening. Most have a shelf life of 2-3 years unopened, 1-2 years after opening. Expired medication may be too weak to be effective, leading to incomplete treatment that selects for resistant organisms.

Dry medications (powdered antibiotics like kanamycin, metronidazole, erythromycin) last longer, typically 3-5 years if stored in sealed containers away from light and moisture. Once mixed into water, use immediately; dissolved antibiotics degrade within hours.

Store all medications in a cool, dark place (not in the humid cabinet above the tank). Label each bottle with the date you opened it. Replace anything older than 2 years.