What Makes an Aquarium Fish Sick?

Fish never become ill without a reason: diseases are always the result of an underlying problem. It may be something obvious, such as a large, aggressive tankmate constantly injuring other aquarium inhab­itants, or it may be something more subtle, such as a fish slowly wasting away from a parasite infestation. Fortunately, many of the common causes of illness among fish—malnutrition, injury, and stress (the most common problem)—are preventable through proper care of your fish and aquarium.

If a fish doesn’t eat enough food, the results can include a failure to grow, a lack of coloration, and increased susceptibility to disease. Preventing malnutrition in your fish requires you to address two main issues: supplying enough of the right kind of food and providing each individual in the tank an opportunity to feed.

Commercially available products provide suitable basic nutrition for the majority of pet fish available today. A high-quality commercial diet is an easy way to provide your fish with the nutrients they need. The right food promotes natural health, growth, and coloration in your fish. Don’t feed only one brand and form of commercial food to your fish: offer a variety of different foods regularly. Alternating between different foods makes your fish more likely to receive all the nutrition they need to thrive.

In addition to commercial foods, many fish also enjoy live foods such as bloodworms, brine shrimp, or daphnia. Research any fish you add to your aquarium to determine which live foods are best to offer each one.

The best way to ensure that all your tank inhabitants get a chance to feed is to include only compatible fish in your aquarium. Aggressive fish sharing a tank with smaller or more passive fish are likely to make it more difficult for the other fish to eat. You can also remove particularly aggressive fish and keep them in a separate tank, for the sake of both the aggressor and the weaker fish.

Injuries can result from sharp rocks in the aquarium, bad netting, spawning activity, poor transportation, or predator attacks (in an outdoor pond), among other possible causes. Obvious signs of injury include missing scales, reddened areas, split fins, and grazes on nonscaled areas such as the mouth. If spotted early, most injuries are easy to treat with topical medications. However, if injuries go unnoticed for any length of time, bacterial or fungal infections may develop that can endanger the fish—as well as other fish in the tank, should the infection have the opportunity to spread.

The best step to take to prevent the occurrence of injury is to reduce the number of objects in the tank on which a fish can damage itself. If predators are the problem in your outdoor pond, consider netting the pond or installing another type of deterrent device such as wire mesh.

In nature, stress can be a good thing because it stimulates behaviors and physical changes that allow the fish to deal with difficult situations that may arise. Prey species, for example, will attempt to swim away or hide from potential predators. Problems arise in an aquarium when a stressful predicament becomes prolonged and a fish is unable to escape it. Long-term high blood levels of natural steroids—a natural response to stressful situations—suppress the immune system, leaving the fish more susceptible to infection and disease. The following are some of the common stressors that can affect pet fish.

Dominant fish attempt to drive out competitors from areas they consider to be their territory, and several species, such as most large cichlids, may consider the entire aquarium to be their territory. Because the other fish can’t physically leave the aquarium—except by jumping out, which may happen on occasion—they become progressively battered and hurt by the aggressor. They may be unable to obtain sufficient amounts of food to grow properly or even survive, and, consequently, they waste away, often succumbing to secondary infections in their weakened state.

This situation is also problematic for the bully itself: kept on an unnaturally high state of alertness and aggression by constant intrusions into its territory, the bullying fish also becomes stressed and may exhibit abnormal behaviors, such as attacking inanimate objects in the enclosure.

Poor water quality is the most obvious and prominent example of a stressor for aquarium fish, and almost any health problem you may encounter in your fish is in some way attributable to this issue. Fish in a properly filtered and stocked aquarium are much less likely to become ill than are fish in an overcrowded, dirty tank. (See How to Maintain Water Quality in Your Aquarium for more information.)