Both filters are closed off from where the water goes into the filter. The 50 gallon one has a cage but the 10 gallon has nothing from where the water falls out of. With hardly any flow coming out of the 10 thats how it climbed in there.lol yeah that sounds like a typical fire eel move. I'm guessing the intake for the filter has the cage at the bottom so he must have gotten in through the return? What I did to prevent that is squeezed a piece of filter sponge in the gap where the water returns to the tank. The water has to pass through the sponge to get to the tank and there's no way for an eel or bichir to get up the return into the filter