Hmm. You could try posting in the oddball section aswell?
I can't think of any particularly demanding fish other than discus (but my brain's a bit fuzzy at the mo).
You could always try brackish...that'd be harder as obviously you'd have to maintain a certain salt concentration.
There are some really pretty gobies that are brackish (also some freshwater ones eg. Peacock goby)
Also brackish (and something I've been debating for awhile), Indian dwarf mudskippers! Can't remember the exact requirements off the top of my head. But they require a very humid very specific environment to thrive. So technically would be a challenge.