Fish DIse ASE

Fish Disease

SymptomsPossible CulpritTreatment
(1),(2) Cotton Wool Effect on the head, body and the Fins.Fungus, Columnarius (Mouth fungus)If you see this disease, can use medication (ie “Anti Fungus”)
(3) Small, white spots on the head, body and fins.Whitespot (Ichthyophthirius)A very common disease (especially in a new aquarium). There are several formulas around, dose the Piranha quickly before the parasite spreads.
(4)Fraying of the Fins and soft Fin TissueFin rot, Disintegrating finsUse medication before a secondary infection occurs, it will take several days before there is serious fin damage.
(5) Reddening of the skin, or red streaks in the fins.Virus, Ulcers (Bacterial infection)With a bacterial disease, use a anti-bacteria or anti-internal treatment.
(6) Dull colour (mucus), detached skin, fraying of the fin.Mucus Hyperproduction.This called by caused by Slime disease (parasite, meds available), poor water conditioners (high Nitrogenous wastes), Fukes, stress.
(7) Worms are visibly seen hanging from the gills, Piranha are gaspingGill Worm (Dactylogyrus)This is another parasite. Use anti-parasite drugs as directed.
(8) Piranha are gasping, on the bottom of the aquarium, red streaks around gillsMost probably oxygen starvation, or a Gill diseaseYou have too many Piranha, an air punp may help. Gill diseases are quite rare, you could actually see the gills rot away: use medication.
Your new Piranha have died – there are no obvious signs of illnessNew Tank SyndromeAdding Piranha to a brand new aquarium stresses the Piranha, and sometimes it can be fatal. Check out the Nitrogen Cycle.
Loss of appetiteAn enviromental problem, newly-introduced Piranha can be slow to feed (stress), certain Piranhaes will refuse anything but live foods.Enviromental: inspect for any manifestations of disease, and get a basic water tester (Ammonia etc). The same with stress.
Scales are raised and the Piranha is bloatedDropsy (Ascites)Dropsy is more of a sympton than an actually disease yet, and is quite common. Most associated with a bacterial infection (cause could be wide – poor water conditions are often the culprit. Meds can be used, but dont always work.

This has to be one of the most Distressing times for a Piranha keeper, and for the Piranha. One day your sitting there admiring your lovely new Piranha and next minute there little white spots all over there body’s or they stop eating all of a sudden for no apparent reason, or they’re fins become all frayed and seem to be rotting. First of all you’ll be wondering what it is, or can it be cured, so in the following pages you will see some helpful information on Piranha disease’s, causes, how to prevent them, some Diagnosis. It should be said tho that most diseases can be prevented from maintaining good water quality.


Q: Why is my Piranha sick and how do I prevent more illness? A: Probably 80-90% of diseases in captive Piranha can be prevented by avoiding stress. Stress weakens Piranhas’ immune systems, leading to increased susceptibility to disease. Actually, diseases and pathogens are almost always present in tanks, but a healthy Piranha’s immune system will prevent them from being a problem. Some of the most common stressors for captive Piranha are:

  • Poor water quality: measurable ammonia or nitrites, or very high nitrates.
  • The water temperature is fluctuating more than 2 deg F/day
  • Incompatible species in the tank.
  • Too many Piranha in the tank (5 PiranhaFish in 10g tank).
  • The tank is too small for the Piranha (foot long Piranha in 20g tank).
  • The water is too warm or too cold for the species (Koi vs. Piranha).
  • wrong pH for species (Piranha vs. African cichlids)
  • pH fluctuations greater than 0.2 units/day.
  • Insufficient cover or hiding places present.
  • Wrong water hardness for the species (Piranha vs. African cichlids).
  • Insufficient oxygen in the water.
  • Improper Piranha nutrition (wrong food, foods not varied).

Keeping your tank free of disease

Q: Do I need a quarantine tank for new Piranha? A: Quarantining new Piranha is a good habit for all aquaria, but is not absolutely necessary for success. Quarantining is simply keeping a Piranha in a separate tank for long enough to be certain that it is disease free. Many beginners do fine without a quarantine tank, and object to the cost of another setup. A quarantine tank does cost more, but if a hobbyist has hundreds of Pounds invested in Piranha, it is cheaper to have a separate quarantine tank than to replace Piranha killed by a newly introduced disease. Also, many of us become attached to Piranha and do not want to expose our pets to diseases from newcomers, no matter what the cost .

The purpose of quarantining is to avoid introducing new diseases to a stable system, and to be able to better observe new Piranha for signs of disease. A quarantine tank can also double as a hospital tank for sick Piranha. Hospital tanks are good because they lower the cost of using medicines and keep diseased Piranha separate from healthy ones. Quarantine is probably most important for saltwater tanks/reef systems because of the difficulty of treating diseases, or wild-caught freshwater Piranha because they are probably not disease-free. Quarantining itself can stress Piranha so be sure quarantine is as stress-free as possible.

To set up a quarantine or hospital tank:

  • Keep an extra filter — a sponge filter is ideal — or piece of filter floss in an established tank, so that you don’t have to keep the quarantine tank set up at all times. Some people choose instead to keep the filter going with guppies or danios (for freshwater) or mollies (for saltwater).
  • If you don’t keep the tank running, use old tank water to fill the tank. So: old tank water + established filter = instant established tank.
  • Add a spare airpump and heater. If you haven’t messed with the heater during storage, it should come to wherever you had it last time.
  • Consider using Amquel or equivalent when medicating the tank in case the biological filter bacteria are sensitive to the medication. Sick Piranha are especially susceptible to ammonia. (Note that ammonia which has been bound with Amquel still shows up on a nessler ammonia test. So, if you are planning on testing for ammonia in that tank, you need to use a salicylate ammonia test.)
  • For a hospital tank, do small, frequent water changes (even every day).

If possible, quarantine all of your new Piranha for about three weeks. During that time, gradually acclimate the Piranha to your tank’s parameters: hardness, pH, salinity, temperature, etc., and watch for and treat any signs of disease. Do not medicate quarantined Piranha “just in case.” Only treat evident, definitely identified diseases. Treating all quarantined Piranha with a bunch of medicines will just lead to weakened Piranha and antibiotic resistant bacteria.

Once you are done with the quarantine, if you treated any especially nasty diseases, it is good to disinfect the tank and reestablish the filter. Chlorine bleach or strong saltwater (for freshwater) work well. Be sure all traces of bleach are rinsed off. Another good disinfectant is potassium permanganate.

Q: How about quarantining plants?

A: Plants can carry diseases into a tank, too. It is a good idea to disinfect new plants if there were fish in the tank with them at the store. There are a few different ways to disinfect you plants, here are a few:

Q: How do I disinfect my plants?’

New plants may have unwanted hitchers: snails, algae or disease. Disinfection can help reduce their transmission into the tank, and can be used to remove algae growths from established plants. Beware, there is always a danger of going too far and damaging the plant itself. Some popular methods:

  • A ten minute soak in potassium permangenate (pale purple) works well; it is available in dilute form from Jungle products as “Clear Water”. Permangenate is particularly good for killing bacteria and pathogens.
  • A 2-day soak in 1 tbsp/gallon of alum is good for killing snails and their eggs.
  • If the plants are kept in a fish-free system for three weeks, parasites like ich and velvet will die without their fish hosts.
  • A soak in a 1:19 diluted bleach solution; 2 minutes for stem plants, 3 minutes for tougher plants. Make sure to remove all traces of bleach afterwards by rinsing with water and dechlorinator. This method can kill your plants, so use only as a last resort against hell algae

Q: How do I avoid introducing diseases in the first place?

A: Never buy sick Piranha from a store. Especially do not buy Piranha or plants from a tank if *any* Piranha in the tank shows any signs of disease or if there is medicine in the water (water is colored yellow, green, or blue). Store people may say the Piranha are fine, but if they were, why is the medicine in the tank? Also ask how long the Piranha have been in the store. New arrivals may be carrying diseases that have not shown up yet. It is better to wait a couple of weeks before purchasing the Piranha. If you must have a Piranha that just came in, be especially sure to quarantine it properly.

Diagnosis/common diseases or: How do I know the Piranha is sick?

Most important: watch your Piranha and know what their normal behavior and appearance is. If you don’t know what normal is, you can’t know what sick is.

Bad signs:

  • Clamped fins (fins are held abnormally close to body)
  • The Piranha refuses its usual food for more than 2 days.
  • There are visible spots, legions, or white patches on the Piranha.
  • The Piranha gasps at the surface of the water.
  • The Piranha floats, sinks, whirls, or swims sideways.
  • The Piranha shimmies (moves from side to side without going forward).
  • A normally active Piranha is still.
  • A normally still Piranha is very active.
  • The Piranha suddenly bloats up, and it’s not due to eggs or young.
  • The Piranha is scratching against tank decorations.

I suggest setting up a Piranha medicine cabinet. It seems like Piranha always get sick when the local fish shop is closed.

  • Water quality test kits: pH, ammonia, nitrite, nitrate
  • Aquarium salt (NOT table salt. Most table salts contain additives to keep them from clumping. Kosher or rock salt is OK).
  • Malachite green/formalin ich remedy
  • Methylene blue
  • Chlorine bleach for disinfection
  • Maybe one antibiotic (Kaynamycin or Furanace)
  • Antibiotic-containing food
  • Copper remedy for parasites

And for Piranha big enough to handle:


Malachite green or mercurochrome

Common diseases/problems or What’s wrong with my Piranha?

Bad water quality

Piranha are gasping at the surface, or very inactive, but there are not visible lesions when it first starts. Their fins may be clamped. Many Piranha of different species are affected, and possibly the whole tank. If the water has been bad for a while, the Piranha may have finrot, or streaks of blood in their fins.

• If Piranha are gasping at the surface, or have purple gills: high ammonia or low dissolved O2 may be the problem; test ammonia, dissolved O2 • If the main symptom is inactivity: test nitrites, pH, dissolved 02, nitrates Depending on your test results, try the following:


Change enough of the water to reduce ammonia levels to 1-2 ppm for freshwater or below 1 ppm for saltwater. If that means changing more than a third of the water, be sure the water you add is the same temperature, salinity, hardness and pH of the tank water. It is also okay to do multiple smaller water changes for a few days. Aerate, and make sure pH is at or below 7.0 for freshwater tanks. In addition to or instead of changing water, you can also add a dose of Melafix to give Piranha immediate relief. Find out why ammonia is present and correct the problem.


Change enough of the water to bring nitrites down to below 2 ppm (as with ammonia, if this is a lot of water, match water parameters or do multiple water changes), add 1 tbsp/gallon salt (not all Piranha may tolerate this much — start out with 1 tsp), and add supplemental aeration. Find out why the nitrite levels are high and correct the problem.


Change water and clean the filter. If your filter is dirty, there is more waste material present to break down into nitrate. Start feeding less and changing water more often.

Low oxygen

Run an airstone. If this helps a lot, the Piranha probably don’t have enough oxygen in the water. Your tank may need cleaning, fewer Piranha, or additional water movement at the surface from a powerhead, airstone, or filter.

Improper pH

If pH is too low: make sure carbonate buffering is adequate — at least 5dKH. In general, adding baking soda at 1 tsp. per 30 gal. raises dKH about 2 degrees. For a 10-20g tank that just needs the pH a little higher, try about a quarter teaspoonful. If that isn’t enough, add up to a teaspoonful more. You can scale this up to 1 tsp/30 gal for larger tanks. If the pH is still too low and the KH is at least 5-6 dKH, clean the tank. For long-term buffering in saltwater and alkaline freshwater systems, add crushed coral. If pH is too high, pH down (phosphoric acid) can be added. Don’t rely on this stuff, except in extreme situations like ammonia poisoning because it can cause excessive algal growth. To lower pH long-term, filter over peat, or use distilled or deionized water mixed with your tapwater.

Freshwater Ich

Symptoms: Piranha look like they have little white salt grains on them and may scratch against objects in the tank.

White spot disease (Ichthyopthirius multifiliis) is caused by a protozoan with a life cycle that includes a free-living stage. Ich grows on a Piranha –> it falls off and attaches to gravel or tank glass –> it reproduces to MANY parasites –> these swarmers then attach to other Piranha. If the swarmers do not find a Piranha host, they die in about 3 days (depending on the water temperature).

Therefore, to treat it, medicine must be added to the display tank to kill free-living parasites. If Piranha are removed to quarantine, parasites living in the tank will escape the treatment — unless ALL Piranha are removed for about a week in freshwater. Some people think that ich is probably dormant in most tanks. It is most often triggered by temperature fluctuations. Remedy: For most Piranha, use a medication with formalin and malachite green. These are the active ingredients in many ich medications at Piranha shops. Some products are Kordon’s Rid Ich and Aquarium Products’ Quick Cure. Just read the label and you may find others. Check for temperature fluctuations in the tank and fix them to avoid recurrences. Use these products as directed (usually a daily dose) until all of the Piranha are spot-free. Then dose every three days for a total of four more doses. This will kill any free-swimming parasites as they hatch out of cysts.

Another remedy is to raise the tank temperature to about 90 deg F and add 1 tsp/gallon salt to the water. Not all Piranha tolerate this.

Fin rot

Piranhas’ fins turn whitish and die back. Fin rot often follows damage or injury. It can also be caused by poor water quality.

Remedy: First, fix the water and remove any fin-nipping Piranha. Change some water (25% is good) and add 1 tsp/gallon salt to promote healing. If bad water quality or an aggressive tankmate was the problem, that should be adequate. Healing will begin within a couple of days. If it worsens, decide first whether it’s fungal or bacterial. Fungal finrot looks like clumps of cotton on the fins and usually follows injury.. Bacterial finrot is whitish, but not cottony (unless it’s columnaris), and can be contagious. The Piranha then need to be removed from the tank and medicated.

Fungus: For Piranha large enough to handle, catch the Piranha, and dab malachite green directly on the fungus with a Q-tip. This is extremely effective. Repeat treatments may be necessary.

For small Piranha, a commercial fungicide such as Maroxy may work. For severe infestations, try a bath in methylene blue (enough so you can barely see the Piranha) until the fungus turns blue or for 20 min. If you add methylene blue directly to a tank, you will kill plants and trash your biological filter.

Bacterial: Antibiotic treatment in a quarantine tank. This is stressful for the Piranha, and doesn’t always work, so be sure of what you are doing before you attempt it. If the Piranha is still eating, the best bet is an antibiotic food. Tetra makes one that works well — just buy the one for bacterial diseases and follow the directions on the can.

If the Piranha is not eating, a bath treatment is necessary. A combination of Kaynamycin and Furanace usually works, especially for Columnaris. Again, treat in a separate tank and aerate heavily.


Piranhas and other “scrappy” Species may sustain injuries that are severe enough to draw blood from fighting. Other Piranha may run into tank decorations, walls, or rocks. Larger Piranha can be netted and their injuries dabbed with mercurochrome or Betadine (iodine-based antibiotic also available at chemists) to help prevent infection. Be sure to keep these chemicals off of the gills and eyes. For really small Piranha, put the affected Piranha in dilute methylene blue (pale blue) and 1 tsp/gallon salt in a separate tank. If you want to keep the Piranha in the main tank just add salt, as methylene blue will trash your biological filter. Watch the Piranha to be sure injuries are healing cleanly, and repeat the mercurochrome dosage if necessary. If finrot or fungus sets in, see the above section on finrot.


Piranha swells up like a balloon and may show popeyes. It may recover with no treatment and may die despite it. The swelling is because the Piranha is absorbing water faster than it can eliminate it, and it can be caused by many different problems. High nitrates are one thing to check. Internal bacterial infections, including Piranha TB, are other possibilities. If there are no water quality problems, you may want to attempt antibiotic treatment in a separate tank.

Head and Lateral Line Erosion (hole-in-head disease)

This disease can affect discus, other cichlids, and many saltwater fish. The fish develops holes in it’s head and sometimes along its lateral line. Causes are unclear but as in any disease, stress and poor water quality likely play a role. The Manual of Fish Health states that HLLE is probably due to nutritional deficiency, especially of vitamin C. Piranha in planted tanks rarely get HLLE, which supports the nutrition idea, since Piranha can nibble on the plants and obtain extra nutrition. Untergasser also observes that the protozoan Hexamita can be found in the lesions. Untreated cases can eventually prove disfiguring or fatal.

Remedy: First, make sure water quality is optimal and reduce stress. Stopping carbon filtration may help as it can remove nutrients from the water. Then feed a vitamin-enriched food, paying particular attention to vitamin C supplementation.

For stubborn cases, some books suggest metronidazole (Flagyl) to eliminate Hexamita (a mildly pathogenic protozoan) from the lesions. Your mileage may vary with that one. Metrozole and Hex-a-mit are commercial medications with metronidazole.

Swim bladder disorders

Piranha floats upside-down or sideways. This is particularly common in fancy goldfish because of their bizarre body shapes. Dry food eaten quickly swells up in the Piranha’s intestine and keeps the Piranha from controlling its swim bladder properly. To help, feed the Piranha pre-soaked or gel-based foods. Green foods are also helpful; peas in particular.

As with finrot, these disorders can also be caused by bacterial infection. Treatment is much the same. Use antibiotic food if the Piranha is eating, or add antibiotic to the water in a quarantine tank if the Piranha is too sick to eat.

Large external parasites (as opposed to ich)

Add a copper remedy to the tank and monitor it with a copper test kit. Also, Mardel’s Maroxy works well. For anchor worms or leeches on pond fish, remove them from the affected fish with tweezers and swab the area with mercurochrome to prevent infection.


Piranha look like they have been finely dusted with flecks of gold. Fins may be clamped and the Piranha may shimmy. Treat with an anti-parasitic medication such as copper or formalin/malachite green.

Edited by Ian.