As I said before, Miy dwarf gouramis tend to do that. Did you get two males? I found that one of my males was, like yours, chasing the other one and was trying to bite the tail. This is because one of my males had decided on its territory and whenever the other one trespassed into it, it would chase it. Perhaps that is what is happening to your fish. Its only really serious if you get some fin damage on the chased one. Mine havn't got to that yet. If you really want to do something about it, rearrange your tank furniture so that the agressive one can't make territories. It helps