Before Sunday, I had 2 fish in my 10 gallon tank, recently upgraded to a 10 gallon from a 5 gallon. So, before Sunday I had a Female Dalmation Molly and 1 Yellow Fancy Tail Guppy Male. On Sunday, I went out and bought 2 balloon female mollies and 2 male fancy tail guppies. Ever since I have added these new fish in, the Yellow guppy has been chasing the new fish and trying to bite them.. Is this normal behaviour? When I had just the 2 fish before, the Molly was usually the more aggressive one but now it is the other way around. The tank temp stays at 78F and it is filtered with a Power Filter. Any ideas why my fish is doing this?