While most fish can tolerate an environment that differs somewhat from their natural conditions, this doesn't mean they will be as healthy, happy, or live to their full life span. Stress reduces a fish's ability to ward off disease and heal itself. Stress also reduces a fish's ability to breed successfully. As stress levels increase, the fish's ability to cope with it decreases. For your fish to be healthy, happy, and live a full life, it is very important to remove sources of stress whenever possible.

Some giveaway signs of a fish under stress include: fish gasping for air at the surface of the water, a fish refusing to eat, a fish that stays hidden continuously, a fish with nicked fins or open wounds that don't heal, a fish that becomes visibly "sick" and loses color, a fish that exhibits "abnormal" behavior, a fish that is unable to control itself in the water. It is important to know the characteristics of the species you are keeping, and their requirements,to be able to identify stress conditions.

Fish will get stressed for many reasons. The level of stress induced is dependent on the species. Factors include:

Fish will get stressed for many reasons. The level of stress induced is dependent on the species. Factors include:

Ammonia, Nitrite, Nitrate	Nitrogen compounds have varying degrees of toxicity and are stressful at all levels. Ammonia is toxic in low concentrations and severely stress fish under any concentration. Prolonged exposure is deadly.
Water Temperature	Keeping water too cold or too warm for a species of fish will cause stress. Wide ranges of improper temperatures will cause the fish's body organs to work harder to keep it alive.
Water Chemistry	Most fish have specific preferences for pH, GH, and salinity. Untreated tap water containing chlorine or chloramine are toxic to fish, and will poison them in even low levels, causing stress, and eventually death.
Oxygen	Fish need oxygen. An overcrowded tank will cause oxygen levels to drop. Insufficient oxygenation will cause stress.
Physical Space	Keeping a fish in a tank that is too small for it will cause stress, and usually lead to aggression towards other tank inhabitants.
Fish Aggression	Many species of fish "don't mix well". A fish that is constantly being harassed or chased by another will be continuously stressed.
Lonely Fish	Most fish prefer company.  Fish that school in nature, will never feel comfortable or safe unless they are in a school of at least 6. Even non-schooling fish will be lonely if they have no tank mates. Other fish don't like competition at all and will be aggressive to members of their own species.
Poor Nutrition	Poor nutrition causes stress. A healthy diet is a varied diet of fresh foods. Picture yourself eating the exact same low grade food day after day...
Medication	Medications that kill parasites and bacteria are often themselves toxic to fish. They often will also affect the nitrifying bacteria.