Hello. I have a 55 gallon tank. I got 6 platies at one point and they must have been sick because after they started dying off, my other fish started dying off and it wiped out more than half of my fish population. I had 4 serpae tetra left, two zebra danio left (one of them recently got dropsy and died from an unknown reason), and one platy left. I am working on restocking and I got 15 serpae tetra about a week ago. But ever since then my platy has been chasing them a lot. Is there a reason why it just started doing this? Should I be worried?