although most of the time fish dont get into the overflow system, at times some types of fish will jump in and get sucked through the U tubes. and if that happens they can get sucked into the sump. I had this happen just this weekend, kinda odd, I was looking all over the place for my six line wrasse and low and behold he was in the sump, wow, I thought either that dude can jump a good four feet and hit a 4 inch opening in the lid or it just got a nice little water slide ride. that was the case the water slide ride. It can happen but it isnt common