Keep them in a half-filled tank. This will make your life a lot easier. They escaping by swimming up to the top and then wiggling into crevices, and once their, pulling themselves along. In the wild they apparently cross wet grass and mud going from pond to pond. So if you lower the water to 50% the height of the tank, assuming the tank is reasonably deep, they can't get enough grip to get into the cracks at the top of the tank.
At university, that's how we kept eels (Anguilla spp.) which are just the worst fish for keeping in an aquarium. It's also standard practise for things like swamp eels, which are also great escape artists.
You can also use fine plastic mesh to build a "clip-on" lid to go over the tank. Make it big enough to cover the top of the tank, plus a couple of inches to make a fringe that you can fold down all four sides of the tank. Tack the four bits of the fringe together so that you make what is in effect a sort of box that clips over the tank. Green stiff plastic mesh can be purchased at garden centres very cheaply (used for ponds and things, a couple of pounds per square meter) and all you need to do is cut with scissors and use something like unwound paperclips to tie together the 'fringe' pieces.
This approach will let you fill the tank closer to the top, because the fish can't escape. You'll need to only use things like filters and heaters that go inside the tank, since at best you can wind air pipes and power cables through the mesh (maybe taking the plugs off to do so). As soon as you cut spaces for water pipes (i.e., for canister filters) you make holes the fish can escape from.
Cheers, Neale
