Yesterday, I noticed there was a guppy with it's bottom tail cut. It was swimming on his side and on the top of the tank, but still swimming. Today, I noticed that he was being thrown around by the current (which was not happening yesterday) so I put him in a small cup to hopefully calm him down. It's been a couple of hours and he's just mostly been resting on the bottom. I was wondering how often I should change the water, what else I can do to help him, and how it might have happened. We did get some new fish a couple of days ago, so someone might have gotten nippy, but I'm not sure. There has never been a problem with fins being cut in this tank before, and it has been used for years. The new fish are male fancy guppies (I think) and some red tetras to go with my neon tetras. I don't know the chemical stuff in the tank, and I doubt my parents will get anything to find out for this one fish. Mostly I was wondering how often I should clean the water in tge cup, but the other things that I mentioned would be nice to know too.