I know everybody says "LARGE FISH! LARGE FISH!" but I think it'd be really cool to get large amounts of decent sized (6 - 12" ) schooling fish...think about having enough space to house a school of tangs! that'd be crazy awesome. of coarse, a 5000 gallon salt would be difficult to maintain...if you'd go fresh water, I'd stick with the same principle...but try and go "different" than most...I love aro's but a african cichlid biotope with tons and tons of medium to large cichlids would be awe inspiring...large fish are really cool, but movement and motion appeal to me more these days...it's gotta have "FLOW" to it...how about putting 500 rainbow fish in it? That would be something....5000 gallons could easily house huge amounts of medium sized fish...think of the possibilities!