90 gallons of water is 750 pounds, not 900. The tank weighs a lot too though, plus the stand and other stuff. Probably around 900-1000 pounds total. I don't think it will be a problem.
the way you have to look at it is can your floor take 4 200lb men standing in one place? if your floor can't take that then you might have to worry, that was my assurance when i put my hundred gallon upstairs, i still put it on a load barring wall
Agreed I have a 125g, two 90g and 55g upstairs in a house that was built in 1989.
The best thing to do is to place the tank so the weight is spread across multiple floor joists not a couple. IE: place the tank perpendicular to the joists not parallel to them. Plus close to a load bearing wall beneath.
the way you have to look at it is can your floor take 4 200lb men standing in one place? if your floor can't take that then you might have to worry, that was my assurance when i put my hundred gallon upstairs, i still put it on a load barring wall
That is not the way to look at it. Even though the floor can support 4 200lb men, they are all not standing in one spot for years.
Said that I still think it is OK. Can you look and see which way your floor joists are running? If they are running perpendicular to where you want to put the tank you are better off. Also try to find out where your load bearing walls are.
The only way to be sure is to get a structural report, doesn't cost too much and gives great peace of mind and guaranteed insurance pay out if anything was to go wrong...
