The MSX 160 would definitely fit, and is an awesome skimmer. I have one in my 65 gallon right now and it is great. I am upgrading soon to a 120 gal, and Will probably upgrade to a msx 200....maybe even the 250...lol
The 160 would handle a 90 gallon with no problems, I just recommended the 200 because it is only $20 more money right now.