Actually I have a prizm, and it skims, though it is a bit noisy. I can't get cpr or aqua c remoras which are both better regarded, nor can I expend the cash required for a little tunze or deltec. It skims, it was cheap and I can live with it.
An eel will go in a 40 for a while as they're pretty sessile, but any trigger will soon outgrow a 40. I'd give a clown or undulate a miss as it seems the violence factor is just too much for what is effectively a little tank. A picasso should work for a while but I've never kept one so am only guessing.
I have no problem believeing a domino damsel could kick an eels butt. They are nasty, and eels are much less so thn you'd expect. Good companions for eels, and triggers are big angels, tangs, groupers but you only have a 40 so these are all no-no's
If you're going to keep an eel in a 40 I think that will be about it long term else you're water parameters will be all over the place. To deal with that sort of bioload you likely will need a wet dry or similar to deal wth the ammonia and nitrite, but you need a ay to get the considerable amounts of nitrate out, or hello algae. Water changes - I can't imagine bothering to install a denitrator on a 40. I suppose you can keep other stuff with a small one, but predatory fish can grow pretty fast, so 'getting a big tank in a year or two' is not much help.
