Everything outside the required environmental- or behavioral needs of your fish causes stress. Stress greatly affects the health, reduces the life span, and increases your fishes' vulnerability for diseases . Stress is also the main cause of a deteriorating slime coat that protects the fish. This slime coat is the defense system toward infections.
Fish can adapt to slightly different conditions in their environment.
Understanding the factors that contribute to stress, will eliminate or reduce unnecessary risks. Stability in the water column and a balanced environment are as important as eliminating some other potential stress causing factors. Compatibility is the key. Aggressive fish chasing others, naturally or as part of a mating ritual or territorial defense. Smaller species without adequate hiding places feel unsafe among bigger fish.