I have an interesting story about an undergravel fliter and some guppies. Everything was going fine and most of my fry would not make it because the fish would eat them. Then my male died and I left females in there. One day when I was replanting a plant I noticed a fish inside the tube of my
UGF. I panicked a little and tried to figure out who was stuck in the tube and how they got there. I did a head count and it was not one of my fish. I opened the tube up and he (adult sized guppy you would purchase in a pet store) came darting out along with some other gunk. I decided that the UGF was just hiding the nasty stuff and I took them out and did a
water change. As I was filling it back up I found ANOTHER guppy. She was larger than what you would purchase at your
LFS. I decided then and there that I would never have another UGF. But the fry somehow found a way down the rocks and under the UGF and they lived there. I know that the female was at least a year old (based on when I had a male last). So one day you may find a little surprise in your tank.