Sometimes external stressors can cause aggression amongst tankmatesFish wounds and injuries can result from a variety of different influences, with tankmate aggression being among the more commonplace. Unfortunately, hobbyists don’t always recognize this problem right away when it manifests itself. Jay Hemdal, Curator of Fishes and Invertebrates for the Toledo Zoo, explains why in the following excerpt from his book The Salt Smart Guide to Preventing, Diagnosing, and Treating Diseases of Marine Fishes: Tankmate aggression This is a potential issue in any marine aquarium that houses more than one fish. In the most serious instances, where the aquarist severely underestimates the ability of one fish to injure another, aggression actually takes the form of predation. There are also cases of fish living peacefully side by side for years and suddenly beginning to fight. Home aquarists are often in denial that any aggression is taking place because, with the exception of the most severe cases, it is very difficult to “catch them in the act.” Take, for example, a case of minor aggression where one fish is “tagging” another one (damaging a fin with a bite) at a rate of once or twice a day. What are the chances that an aquarist is going to see the aggressive act?