Ya its crazy how they can fit through at times. I have 1/4" netting,so nothing but some gobies should fit....I think it could have hit a corner and lifted the top to squeeze through. I only have a few that jump often and the netting usually does the job. This rhomboid wasn't one that usually jumps. Was larger than most also, so could have been large enough to move the whole top. Sucks anytime you lose a fish especially when its one of the favs in the tank.It doesn't take much, or anything, for a fish to go right through a hole, even if it's small. I have mine tripled over, at angles, like a diamond, and I just had a fish go right through it, and from the size of the fish that shouldn't have happened.