What are the I-beams spaced at(19"centers)??? Make the stand with a wide ledge(like 4-6") around it to distribute the weight over a wider area. If you can get the weight over two of those I-beams then Bob's your uncle.

I can relate to this problem, my 75G isn't going to be see water until we move house to somewhere where the joists aren't 2x6 or isn't rented so I can beef up the floor!

I think that's the way to go if you can't beef up the floor. If I remember correctly it said somewhere that the tank was going to be over 20" wide, so if they're 19 and a bit" centres, you're hitting two for a start, a 10" ledge on each side means you'll hit three. It could take some careful positioning of the stand so you're actually over the joists themselves and bearing directly on all three. The subfloor should also help to spread some of the load to the rest of the floor.

Given that there's access to the joists, the manufacturers name is usually printed all over them, so you could try and phoning them. If you can tell them how many joists you're over, what the load is and how far out from the bearing walls and/or centre beam of the house they should be able to tell you if it's OK or not.

Just my $0.02

Rob.

I have been reading up on TJI joists (i-beam joists) and it seems the general consensis is that it will hold. If I go with the 48"x24"x16" I should have anything to worry about. As for the stand, after taking your guys advice Its probably going to be 26" wide with a base to distribute the weight (thanks for the idea btw, very useful). At that size of a tank, I haven't read of any issues at only 80 gals. Than with an additional 20ish in the sump. Than again i am going to be measuring and taping off how it will sit in the house so the size isn't 100% confirmed, really i just want more area for corals to mount. having both long sides should make me a happy camper either way.