Eels are VERY strong.
A large adult can easily push its way out of a poorly covered tank. If you want to keep eels in a tank, you may want to start out with juveniles. The best way I know to get elvers is to find a shallow muddy cove with lots of logs and smaller pieces of wood near the shore. Walk around and flip over every piece of wood you can. Be ready with an aquarium net to scoop up the elver as he tries to head for cover.