My brother is a marine biologist... he keeps 2 mandarins in his 25 gallon Eclipse... we just had a big discussion on whether it was a good idea to use an Eclipse in marine fishkeeping... this is what he says WRT the tank:
Can you keep a marine tank in an eclipse? Yes.
Is it recommended by most people? No.
But you get some people who say,
"you need x, y, and especially z, if you're ever going to keep a marine
tank at home."
I think it's a lot of bologna as long as you maintain your water quality with water changes and most importantly....don't overfeed!
My Eclipse has carbon mechanical filtration, and a biowheel, and other than that only about 10-15 pounds of live rock, and I have never had any nitrites or ammonia.
The only thing I wouldn't consider keeping in the Eclipse are corals
which may not get the appropriate light.
So to answer your question, can you do it in an eclipse? Of course you can, I've got the proof at home.
I guess it also depends on what you want to keep. Some fish of course are going to be easier to keep in an Eclipse than others, but I think they work pretty well overall if you are choosing a small system.
Hope that helps...
Katie