The eel would be fine especially if you are really into eels. Just make sure that you do everything in your power to make escape impossible.
I used to have large tree frogs that would make a run for it everynight. But after analizing all possible escape routes, I was finally able to seal the tank well enough to keep them in with no problems. Eventually they found that they could no longer get out and they gave up. 8)
Hardware: 37 gallon SW
tank and stand by Oceanic Systems, Magnum 350 Pro Canister Filter (half carbon), CPR Bak Pak 2R skimmer, Maxijet 1200 Powerhead, RenaCal 150 W heater, Compact fluorescent lights with moonlights. 40 lbs of liverock, 3" sand bed.
Software:2 Tank-Raised Ocellaris Clowns, 1 Purple Firefish, 1 Electric Orange hermit crab, 18 Blue Legged hermit crabs, 8 or so Nassarius snails, Xenia, Blue, green, and Green Striped Mushrooms.
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Central Illinois Marane Aquarist (CIMA) member.