best filter for first time 90 gal. tank owner

First, I will say that stainless is just not necessary. If you coat it well, powdercoat, Hammerite, etc you just will not have rust problems. Clean welds and sealed joints are of course important. .120 wall is good for almost any part, but use rectangular tubing on edge for any horizontal stringers or joists. Square or round is weaker. Just adjust the size (not the thickness) for larger tanks. A center support is a must on anything over 4 feet.
For a 90 gal 1.25 square tubing posts and 1x2 stringers will be adequate. You can weld nuts to the bottom of the legs for leveling bolts. A metal disc or large washer welded to the bolt head and contact cement dome neoprene will give you a nice foot.
A must is some sheer support on the back to keep it form folding if someone pushes real hard on the end. A couple of 45 degree pieces will be more than ample going from top rail to the bottom one or sides. If you weld the botom rail a few inches off the ground, the space makes for easier water cleanup underneath should your tank fail.
Other than that, you can do panel insert doors with magnetic attach points and other neat stuff. Metal allows for a lot of imagination.

 

do it victorian style, like steampunk just not stupid.

 

Love the input;DIll be using .125 2 by 3 tube aluminum. I have alot.I think Ill put it on large casters.I like the idea of raising the bottom shelf and having it roll out.I think Ill enclose with nice hard wood pannels or doors

 

Yeah, 2x3 Al will be plenty. no center support needed. easier to screw things to. I like the drawer idea, but plumbing around it will be a trick. I would still recommend some way of leveling the casters. You would need a decent thickness of Al (3/8" probably at least) on the leg bottom to tap for a heavy duty bolt welded to the caster.
Happy tigging.