What are some common causes of stress in fish?

Like most cold-blooded pets, the health of pet fish is closely linked to their environment. Water quality is the biggest cause of stress and mortality in aquarium fish and is often overlooked. There's often a tendency to equate survival to happiness and given that the animals have no choice but to adapt to conditions, elevated levels of nitrate and phosphate are often endured but can cause low-level stress and long term health issues.

Combination of different species can often be a source of stress and this can take a couple of different forms. As many community fish are naturally social shoalers it's important to keep sufficient numbers for them to be comfortable and display natural behaviours. For more feisty fishes such as some of the barbs, a lack of sufficient numbers means that those at the bottom of the pecking order can suffer a lot of harassment. Other classic compatibility issues are those involving pairs of cichlids taking over community tanks during spawning, or mixing marine fishes with widely differing levels of territorial aggression.