Yeah. We had a fish go round with a square head as it got stuck in the filter and couldn't remove itself. It lived for ages after just looked really weird.
The other possibility is that they are eating each other. You may find skulls on the bottom in corners. Depending how big the tank and the fish are in relation to the tank, you may just have to look harder to find them. We have natural coloured gravel in our tank and 'lost' a small neon not long ago. I found it eventually, hidden under a small pebble.
Good luck with your hunt,
C |