I-joists are stronger, but that just means they will use less of them to save money. In my 9-yr-old house I have a bunch of 12" silent-floor-type joists 24" apart on the main floor. My 150 gallon is going to span only a couple of them, even being set up perpendicular. Good thing it's goiing up against a load-bearing and an outside wall both.
This article from a structural engineer might help:
http://www.cichlid-forum.com/article...ium_weight.php