definitly dont do a heptus tang, they are sensative and can reach up to 16 inches in the right conditions. a 90 gallon just isnt big enough. as far as a moorish idol idk who told you they get 4 inches, the smallest ive seen has been about 5 inches and the largest ive seen has been about 8 inches im sure they can get larger than that, but thats besides the point, moorish idols are best left in the wild as are alot of fish species, some fish just suitable for the aquarium and often die and quickly. if you like moorish idols id suggest a banner fish which is in the butterfly family, they look very similar but the banner fish stays small enough for the 90 and is very hardy but if this is a reef tank then stray away as they may nip soft corals and clam mantles. a yellow tang is a little bit of an aggresive tang and can reach from, what ive read 15 inches and yes they need swimming space. a scopas tang is the best choice here as they a more peaceful than a yellow tang i believe they only reach 12 inches but again swimming space is needed, but if you get it when its small i believe you can keep it alive and well in your 90 for its life span. just make sure to feed any of these choices which ever you go with a good source of spirlina algae as this is their natural diet, without algae your tang,m rabbit fish, or buttefly fish will slowly die. first they will start to fade in color, then they may get white blotches on the body, in some cases they may get hole in the head disease or later line disease. be sure to offer algae at least twice a week to keep them healthy, but more often is better. ocean nutrition makes garlic enhanced green spirliana algae strips. grab a pvc pipe, pretend your back in college and roll up some algae like a tight joint and rubber band it to the pipe and drop it in. this will give the fish the feel of its natural feeding habbits of tearing algae from rock. hope this helped out- matt
