Not too terribly long ago, I found one of the yellow-head eels on the floor. He was alive and recovered completely from his experience. The next day, I took off the glass tops and made it all an egg crate top. I secured the eggcrate and it was tight against the tank...all except for one small square that I used for feeding. This is replaced after each feeding and a brick placed on top of it. Apparently, it was Mr Eels time to go. He dislodged the brick and as near as I can figure, pushed the eggcrate enough where he was able to get out and the eggcrate "hatch" fell back into place. I found him on the floor, dead. The eggcrate looking normal but the brick was on the ground. At first, the brick not being there didnt dawn on me, but as I was peeling the eel off the floor, I noticed it was not where I put it when I feed (on the fireplace). So, that is my best guess and sometimes fate steps in and waylays our best plans.