How to Observe Your Aquarium Fish’s Health

With most types of illness, fish exhibit visible signs that they are afflicted – such as physical symptoms or changes in their typical behaviour or movements. Keen observation of the fish is vital when it comes to recognizing these indicators. Are they swimming normally? Do they seem sluggish?

Study the fish closely every day when you feed them. Are they all eating well? Healthy fish are generally hungry, eating small amounts through the day, although they may not eat for a short period after a move. There are various reasons why a fish may not eat, but if unresolved, then the fish will become weaker over time and more likely to succumb to fungus. Also note if any fish spit out their food immediately. This can be an early sign of ‘mouth fungus’. With some species, it may be hard to tell how well they are eating, especially with catfish and other nocturnal feeders. Try feeding these fish last thing at night, with the aquarium lights off, to see if they show any interest in food.


A fish is only likely to stop swimming in the terminal stages of an illness, when it will then simply float at the water’s surface. You may notice some difficulty in swimming beforehand, however, depending to some extent on the nature of the illness. If the swim-bladder, which controls the fish’s buoyancy, is affected, the fish will be unable to swim normally. You may see that it cannot dive effectively, tending to float to the surface.

With tropical fish, any sudden chilling of their environment can have this effect temporarily, so your first step should be to check the water temperature in the aquarium. If it seems that only one fish is affected, this may be simply a sign of old age, especially in the case of livebearers such as guppies (Poecilia reticulata).

A fish that repeatedly rubs itself against rocks and other decor in the aquarium is likely to be afflicted by some kind of irritation on its body. Poor water quality may affect the fish’s behaviour in this way, or external parasites may be a possible cause.

Nutritional problems

Thanks to the development of well-balanced formulated foods, cases of nutritional diseases have now become very uncommon. Even so, do not use fish food past its recommended ‘use by’ date, or the fish could possibly start to suffer from a deficiency of certain key vitamins. This may cause them to swim in an unusual way, or show other, more specific signs, such as haemorrhaging at the base of their fins. If you buy their food in cardboard tubs, make sure that these remain dry, because the food could turn mouldy if the base becomes at all damp. Fortunately, most brands of formulated fish food are now packed in waterproof plastic containers.

Setting up a hospital tank

While the idea of setting up a separate hospital tank may seem over-cautious, it may be very valuable, both in helping a sick fish recover, and in minimizing the possible spread of disease or infection within the tank. Since most infections spread rapidly through water, it is advisable to separate the patient to this quarantine tank as soon as possible.

For a hospital tank, the aquarium need not be very large, depending on the size of the fish, and it should be sparsely furnished, with a bare floor. There is little point in setting up an undergravel filter, partly because the filter bed will not have time to mature, and also because the majority of medications used to treat fish are likely to harm the bacteria in the filter bed. It is better to use a simple sponge filter instead. The use of carbon in a filter is also not generally recommended for a hospital tank, because it can inactivate certain remedies, and may itself be adversely affected as a filter medium by others.

Do include some basic decor in the tank, to give the fish a sense of security, using weighted plastic plants which can be easily disinfected and will not require lighting. Depending on which species of fish are in the tank, you may also want to include one or two retreats or refuges, such as a broken flowerpot.

Once you are sure that the fish has recovered, you may return it to the main aquarium, transferring it with extra care to minimize stress. Carefully strip down the hospital tank, and wash all the components with an aquarium disinfectant, so that it is ready to be used again if necessary in the future.

Precautionary measures

If you suspect that the fish may be affected by a fungal problem as well as a bacterial or parasitic infection do not be tempted to use two remedies simultaneously, unless you are certain that it is safe to do so. If you combine medications, you could even inadvertently poison the fish instead of curing them. In their already weakened state, this extra strain may well make them succumb completely. If you suspect you might need to use a combination of commercial remedies, check either with the store or the manufacturer; alternatively, you could seek the advice of an experienced fish vet.

Many commercial remedies used for fish ailments are dyes, such as methylene blue, which may well stain the sealant in the tank. This unwanted side-effect, in addition to the risk of inactivating the filter system (if charcoal or biological, killing off beneficial bacteria), means that it is usually unwise to add remedies to the water in the main aquarium. It may be better to strip down and refill the main tank instead, to try to eliminate any infection.

Filed Under: Pets & Animals


About the Author: Fred Goodson has a passion for pets and animals. He has 4 dogs and is planning to have another one. He is also a blogger who writes about pets and animals. Currently, he is living in New Jersey.

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