my tank has the corners cut for piping/wires etc etc. I don't think any of my other fish would try and escape as its above water level (obviously or i'd have a big puddle!!) . however as the bristlenose scales the sides of the tank, is there a chance he could go above water level and fall out? if they don't ever go above water level (even for a second) then that's great. if there's any chance at all or escape then I'll look at putting some sponge in the gap or something?!