Yeah, I wouldnt feed them rotten fish for the simple fact that it smells pretty much the same when it comes out the other end, but it definitely won't hurt them. If your tilapia is fresh it will be nice and white (except for a darker stripe where the spine(?) was) and it definitely won't be slimy and has a barely detectable fish smell. Shouldn't smell strong, be dark, or be slimy. just wet.