My guppy is hiding behind the filter is he ill???

Abut four days ago I got three guppies in a brand new tank that I set up and left running for a few days. One guppy died so I got another one. Then one of the orignial three got eaten. I just got two tetras and a pletra to add because I was told that they would make good tank mates. One of the guppies is now hiding in the small crevice between the filter and the tank. Is there something wrong with him???