My tank has been set up for about 6 months (new tank) and the water statistics are fine (0 in ammonia, nitrate, etc. and 7.2 pH).The fish include two fancy male guppies, 2 cory cats, 2 dwarf gouramis, a red wag platy, and a rainbow shark, as well as two live plants. All fish except the shark are at their adult sizes (1-2 in. respectively), and the shark is about 2.5-3 inches long. Anyway, when I first got my platy, he started by swimming down to the bottom of the tank, near my shark. For the next few days, he would actually nibble at my shark's fins (at the time, the shark was actually twice as big). After a week of this, the shark began to chase my platy away. Whenever my shark sees the platy, he will chase the platy back to the top and around the tank. After this last water change, he seems more peaceful, but at least once a week, the shark will chase the platy whenever he sees him. They seem peaceful now, but I'm still worried about what caused this behavior, and if it could return. Honestly, I believed that platies were nice, but I'm still worried about my shark's health. (the shark would NEVER nibble on any fish. Though he would chase the platy, he never left a bite mark on any of the fish.) The problem is solved for now, but I don't want my platy to turn violent again. What caused this behavior, and could it reurn? If so, what could I do to save Finn (my shark) without hurting my platy?