A couple of months ago my son bought what he thought were 3 Tetras. 2 were in fact Cornish Rock crosses. For about a month now they have had no feathers on their rears. Is this normal? I have bright pink butts looking back at me. Their bottoms are also pretty nasty looking. It seems that as they potty it all just runs down their bottom or sticks to the bottom or something. Just nasty gross. My son noticed yesterday that while they are pottying they make a strange sound as if it is difficult for them. I noticed this morning that they seem to strain while doing this. What could be happening and what do we need to do? Thank you.