When I started the tank I had 4 monos. They died out and since the scats and the other fish grew so large, I never added more. I feel I owe them a larger tank now. These are some of the most active fish I've seen outside of my african cichlid tank.
Concerning the diameter of the eel. I have (had) three. The smallest is about as wide as, hmm... a nickle and the largest about a 50 cent piece. One recently died, and I found she was loaded with eggs.
I need to build a proper cave for the eels. In the first picture you can actually see one of the larger eels squeezing out of the plastic root. They eat 2 inch crayfish and goldfish up to 2 inches as well.
The funny thing is, whiles eels have a notorious rep for being escape artist, mine have never tried. And I have open slots on the backs of my hoods where hang-on filters used to reside. So maybe they only try to escape when the water conditions are not pleasing. Who knows.