I am just re setting up my 90 gallon tank,the fish I have in it are, a harlequin tusk, a grren bird wrasse, a hippo tang,and a banded snake eel. You can keep aggressive fish in a 90, just not really aggressive triggers, or anything that is going to get too big. I plan on putting in a masked puffer,a volitan lion, and a emperor angel in my tank over the next few months. Dont liston to people that will tell you that you can only hane 2 or 3 nice size fish in a 90 gallon tank, their thinking of full grone fish, and going by the stupid wrong rule of 1inch of fish per 5 gallons. You have to create a balance in the chemistry of the your biological filtration and the amount of fish waste. As long as you have a good protein skimmer, and suffeciant filtration you will be alright. My filtration consists of a Hang on Berlin Turbo skimmer, 100lbs. of live sand,120lbs. of live rock,a uv sterilizer,a wet/dry(that I only use for a sump,I took out the Bio Balls),a aqua clear power filter,and 5 270gph powerheads. This is the same way the tank was set up before and I never had a single problem. The thing that I have seen that works the best in this hobby is to stop listening to people that think they are experts and go with what works for you.