Frequently asked questions on treating sick fish


Fish disease outbreaks need to dealt with promptly, but how do you know what it is? Andy Gordon answers some of the most common questions on diagnosing and treating common aquarium fish diseases.

A fish has died unexpectedly, does this mean there is a disease in my tank?

Even the best kept fish can fall victim to ill health. It doesn't necessarily mean that there is anything wrong with the aquarium when this happens and one of the worst things that you could do in this situation is subject a tank full of fishes to a whole battery of inappropriate medications. Some medication is toxic to some fish and to some invertebrates, adding these medications could easily make things worse.

It could be that a fish has died from an age related illness or trauma (injury) in which case there is absolutely no need to treat the aquarium at all since neither condition is contagious and the only victim is already dead.

Before using any medication in an aquarium or pond, the first thing to do is to try to diagnose exactly what the problem is. This can be done by carefully observing the fish and then listing all the symptoms. You should be able to determine the probable cause of the disease by reading about the most common ailments in this article.

Once you have the diagnosis and you are reasonably confident that it is the right one you can choose the right medication using the Treatment Finder on this website.

What do I need to know about medication before using it?

  • Unless specifically advised to do so, never use two or more aquarium remedies at one time. Different remedies can still contain some of the same ingredients and if two remedies are used it could lead to an accidental overdose of some ingredients and this could be toxic to the fish.
  • Check what the realistic volume of your aquarium is allowing for displacement and the fact that it isn't filled to the absolute top. Failing to do this could render the treatment useless because you won't be using an accurate dose.
  • Check that you can use the medication with all the species in your aquarium or pond. Some species don't tolerate medication very well and in some cases its use can be fatal. Mormyrids (Elephant noses), Orfe, sharks and rays, loaches, corals and other invertebrates are all known for their lack of tolerance of certain medications. Always read the instructions before dosing, and before purchasing the treatment.
  • Some medications are specific to either freshwater or saltwater. These are only intended to work against specific ailments found in that environment and because of this they will at best be ineffective if used wrongly and at worst they might even be toxic.
  • Some medication is toxic to humans, take care when handling these products and wear some protective gloves to prevent accidental contact with your skin.
  • Prompt treatment is usually more successful than treatment which has been put off for a few days. Attacking infections during the early stages is much simpler than trying to tackle a well established infection.

What kinds of diseases might I come across?

There are lots of organisms waiting for the chance to attack a fish. They can be split into different groups each requiring a slightly different approach to treatment and after care. They are: fungal diseases, bacterial diseases, and parasitic infections.

What are the signs of a fungal disease?

The spores of various fungi are always present in an aquarium. Normally fungus doesn't attack a healthy fish because the mucus which covers a fish fights off infection. But if a fish becomes injured through bumping into a sharp object, rough handling or an attack by another fish, or if it has become stressed due to a poor environment, then the fungus could take advantage and gain a hold. Once the fungus as gained entry past the fish's natural defences it will spread and weaken the fish further unless treated.

How will I recognise fungus on my fish?

Fungus (see top picture) is often given the common name of 'cotton wool disease' this is because the fungus looks just like cotton wool attached to the fish. It can vary in colour slightly from almost white to grey, it may even appear green if algae gets in amongst it too. Although this doesn't look too serious it actually is. Under the 'cotton wool' the fungus is eating away at the fish's tissue, so fast treatment is essential.

How can I tell if my fish have a parasitic infection?

Protozoa are the most common parasites that we come across in the aquarium. They are microscopic organisms and part of their life cycle is spent on, or even in, a fish. They can be very infectious in the confines of an aquarium where new hosts are easy to find.

Most are easy to treat, and if treatment begins early enough the prognosis is generally quite good. Protozoan infections irritate the fish and make them itch. This results in a lot of the fish scratching against objects in the aquarium and is one of the first signs to look for when trying to diagnose a protozoa infection, along with rapid breathing and clamped fins. If the infection is very heavy the fish may become quite lethargic and if untreated most will weaken the fish to the point of death.

What kind of protozoa infections am I likely to see on my fish?

  • White spot: If your fish have white spot you will see lots of pin head sized white dots spread randomly all over the fish's body and fins. All parts of the fish will be affected, this is important because you may notice some tiny white dots on your goldfish and assume that it has white spot but that might not be the case. Male goldfish develop tubercles at certain times of the year, these show that the goldfish is in breeding condition. The tubercles will be restricted to the gill covers and around the pectoral fins and no where else, obviously this requires no treatment.
  • Velvet: Velvet is another protozoan infection. Unlike white spot it is less noticeable because the spots are much smaller and more closely packed together. The velvet parasite uses photosynthesis for part of its nourishment and it contains chlorophyll to achieve this. The chlorophyll in the protozoa gives infected fish a distinct appearance and infected fish have a characteristic gold or yellowy green caste to them.
  • Slime disease: There are several protozoa responsible for causing slime disease. As part of their natural defence fish produce mucus to fight off external parasites. When a fish is infected with the protozoa which cause slime disease you will see lots of excess mucus being produced. So much is produced that it may be seen hanging from the body and it is this which the condition is named after.
  • Neon tetra disease: Despite the name this parasite can infect lots of tetras, rasboras, danios and other small fish. Unlike some other protozoan infections which remain external the protozoa which causes Neon tetra disease (Pleistophora hyphessobryconis) attacks the muscle and tissue of the host. Some fish appear to develop a natural immunity to this infection after being exposed to it. Once symptoms can be seen then there is not believed to be an effective cure. The features of this disease are very slight to begin with, there may be a small area where there is a loss of colour. This is followed by localised wasting and finally by a kink developing where the wasting was, shortly after this the fish will die. Fish showing symptoms of the disease should be isolated to help prevent the disease from spreading.
  • Marine ich: This looks very similar to white spot seen in fresh water fish and the two parasites are closely related. In a fish-only aquarium this is relatively simple to treat, but in an aquarium containing invertebrates it can prove very difficult. Buy only quarantined fish since prevention is the best option.

Are my fish suffering from a bacterial infection?

Bacteria are ever present in both ponds and aquaria. These are mostly harmless unless the fish are stressed in some way, as stressed fish fall victim to all kinds of infections more easily than they normally would. Any of the following could be an indication of a bacterial infection: open sores, ulcers, sudden unexplained deaths, frayed or blood streaked fins, areas of raised scales, cloudy eyes, pop eye and wasting.

What kind of illnesses can bacteria cause?

  • Ulcers: Ulcers are deep open sores on the skin. Strangely, affected fish seem to be able to carry on as nothing is wrong, but don't assume that this is the case. Ulcers are very serious and life threatening. They can lead to osmotic problems for the fish so particular attention has to be paid to the overall environment along side any medication.
  • Fin rot: Fin rot usually occurs when a fish is kept in poor conditions. The fins and tail become streaked with blood and they begin to fray. The tail is usually most affected and will gradually get smaller as it rots away. If the infection reaches the body the result can be fatal. Look out for frayed fin edges (not one simple split), reddened or blood-streaked fins.
  • Mouth rot: Mouth fungus or mouth rot, despite its name, is in fact a bacterial disease and is normally restricted to the mouth region. The bacteria responsible form long strands in large colonies which makes them look very similar to fungus. If you see a case of fungus which is restricted to the mouth region then it is most probably mouth rot and should be treated as such, and not with a fungicide treatment. Prompt treatment is required if the fish is to be saved.
  • Wasting disease: Wasting disease is also known as Fish TB. The infected fish can display a whole range of symptoms with wasting, pop-eye and ulcers being the most common signs of infection. This can be a stubborn disease to treat and typically needs professional advice and a course of antibiotics. Veterinary help should be sought if you suspect this condition because under some circumstances it can be passed on to humans. Although not fatal it will require a long course of antibiotics.
  • Pop-eye: As its name suggests pop-eye can be recognised quite easily because the eye swells and stands proud from it's socket. Dramatic as this looks if the cause is fixed the eye will return to normal, often with no treatments being used. There is more than one cause for this symptom. It can be caused by a bacterial infection or simply through the fish being kept in a poor environment. By poor environment this could be poor water quality (ie. pollution) or the wrong water chemistry. Before resorting to an unnecessary course of treatment it is worth checking the fishes environment and correcting any issues there. If the environment remains poor then any treatment is likely to fail.
  • Infected wounds: Occasionally small wounds and abrasions can become infected. You may notice a reddened area or a small area of raised scales or even just a single scale. If the fish is other wise healthy then prompt treatment is usually quite successful.

It is worth noting that well established bacterial infections may only respond to proper antibiotics prescribed by a vet. Early treatment is therefore essential in order to have any chance of a successful outcome.

I have some inverts will they be safe if I medicate the tank?

In some cases, no. Marine and freshwater inverts can be harmed by some of the ingredients used in some treatments, particularly copper-based ones. Some manufacturers take this into account and make an alternative medication which is safe to use with inverts.

Are there other ailments which aren't covered here and if so why is that?

There are other diseases and ailments such as viral diseases and physiological disorders for which there are no treatments available. Some common viral diseases such as Lymphocystis don't require any treatment because after showing some symptoms the fish usually develops a natural immunity and the condition clears.

Other conditions don't respond to simple treatments. Organ failure, tumours, or internal damage obviously can't be repaired with the addition of a few drops of some aquatic potion. Either seek professional advice or consider euthanasia if the fish is really suffering and has no quality of life.

Is there ever a time to when not to treat?

Yes, in some instances when a fish is sick beyond any hope of recovery i.e. when it is suffering with advanced tumours or other incurable disease and it is obviously suffering then the kinder thing is to intervene to end the suffering. Look at it as a final act of kindness for an old friend.

How do I select the right treatment?

Once you have made your diagnosis, you need to treat promptly, ideally after testing the water and correcting any water quality issues. Selecting the right treatment can be tricky, particularly if your tank houses invertebrates or sensitive fish that may react badly to certain medicines.

Where can I get more help?

You could post your query, along with a photograph of the fish and some water test results, on the PFK forum, where our members will be happy to help you. Alternatively, you could seek further advice from a specialist retailer, fish health professional or vet with experience in fish.

This article was written exclusively for the Practical Fishkeeping magazine website.