Thanks Ryan.
Problem is that once he goes into the overflows he doesn't come out -- ever -- he only came out this once because I was dealing with ammonia and he is not as hardy as the gars. So I am keeping him out of the 500g until I can find a solution... my past efforts he got through in a night... he is very smart and does not give up easily!
If you watch the video on page one you can see the eel returning to the overflow immediately (though I had them blocked, hehe), after spending about two months in a separate tank. Very good memory. He got back in that night.