Fresh, marine, brackish eels escaping?

I just purchased an eel for my 55 gallon tank. The lady I spoke with when purchasing it told me to block off all holes in the top of my tank because it could try to escape. If it did escape, how far could it "slither" before dying, or would it just fall near the tank? Thanks, Angie... <Depending on the species, how hot, humid the area is, the type of flooring... a few to several feet, a few minutes to several hours. Keep that tank covered. Bob Fenner> 300 gal tank for eels I was wondering if the following filtration is adequate to keep an eel. Starts out with my 300 gal aquarium, has 2-2” overflows down to 2 separate 100 micron socks, emptying into a 60 gal sump. << So far I'm going to say yes. >> An in sump Kent Nautilus TE Protein skimmer driven by a mag drive 700 sub pump. 2 pumps plumbed into the sump. One drives the reverse flow under gravel filter (bio filtration) that covers the entire bottom of the aquarium and turns over the combined volume of aquarium and sump 3x an hour. ...  more