Is it ok to have a 75 or more gallon tank sitting on carpet, or do you think a hard wood floor ext, would be better. Also do you think i should add support under the house where a large aquarium would be.
My 90 gal is on carpet, I cut a piece of masonite(any flat material will work) to fit under my stand just to transition the carpet to the tank stand. Many people have had good results just setting their stand on the carpet - just don't put it too close to the wall otherwise your tank will lean forward because of this thing they put around the edge of your walls.
As for the extra support - if you can do it go ahead it won't hurt anything and your mind will be more at rest about a 800(est) pound force pushing down on your floor.
make sure to place the aquarium so the beams in the floor are perpendicular to the length of your tank. - it will have more support this way.
Depending on where in the house the tank is in relation to a load bearing wall will determine if you need to add support from beneath. You should have the tank positioned so that it sits perpendicular to the floor joists and not parallel. The more joists the tank rests on the better. A concrete floor would be best but isn't always attainable.
If you want to add support from beneath for extra peace of mind then by all means go for it.