Most Difficult but Ecologically-Sound Fish

Hello everyone,

I'm not an aquarium owner, but actually, I'm doing research for a story which involves aquariums. I know it may be a little off-topic, but I was wondering if somebody could give me some advice.

My question is -- what are the hardest-to-take-care-of fish species which are, nonetheless, ecologically responsible (and thus considered ethical) to own?

I've checked this thread -- http://www.arofanatics.com/forums/ar.../t-143027.html -- which has a lot of listings of difficult fish. But I'm not sure what fish straddles the line, being "hard to take care of and demanding" but not "irresponsible to own." A fish an expert or specialist might own at home, but which would nonetheless take a lot of care.

Also, the fish has to be fairly small... a fish that doesn't weigh more than a few pounds.

I considered the Moorish Idol, but there's so many posts in various forums and sites saying that it's considered irresponsible to own due to its delicacy, I wasn't sure if it's the best fish. The character who will own it, in the story, is an icthyologist, so they'd know how to handle it... but as an icthyologist, they wouldn't intentionally own a fish which is rare or cruel to own.

Any advice would be incredibly appreciated! Many thanks!