One time I was asked to cover a shift at another petsmart in our area. When I looked into the sump, there was a little neon swimming around in it, and I saved it. It looked very very small, maybe they bred? They also had lots of small snails throughout the system. I think the overflow pushed them all through the system! This store has been here for a few years and is very well established.
At a LFS around here that is where they keep all the good stuff is the sump! One time the cover was off so I took a peek and there was a big 12"+ albino barramundi and a big ass wolf fish. In another one I saw a 10" albino bichir and a couple big cichlids. I asked em if I could buy some of them and they said NO and hurried up and put the covers back on?????
I have had some go down the overflow to the somp also. I never was able to find out what they were. I put them back with their mommy (and the rest of the african cichlids) and they must have gotten eaten.