At that size, they are strong and can maneuver their way out of a net with no problem. When I kept one, I would use a large net, and as soon as the eel was in, would twist the net closed, then hold it like that with my free hand to make sure it didn't get out. Don't be timid as the eel will bve fighting like mad to escape so hang on and know it can't hurt you. If the tank is large enough, get a good sized pail with a lid and herd the eel into the pail, sealing the lid after it to hold it in. Either way should work fine.