Water weighs approximately 8.35 pounds per gallon. So 75 gallons would weigh 626.25 pounds. The average refrigerator weighs 400 pounds, and people don't usually check the floor before buying a refrigerator. So I think the floor will support it. As a safety measure, you can be sure to distribute the weight over a flat surface instead of putting the aquarium on something like a table with 4 legs. When you have the 4 legs, each leg is exerting 1/4 of 626 pounds (156.5 pounds) on the floor. But if you put a rectangular board (or equivalent) under the table legs, the weight is spread out over the whole board. For instance, if the aquarium table is 12" x 24", that's 288 square inches. That means each square inch is pressing on the floor with only a little over 2 pounds (626 divided by 288).

Try to spread the weight as much as you can and place the tank as close to the ends of the joist as you can and not in the middle :)

Hi kirababe, There are a number of factors involved. Typical floor loading is probably around 40 pounds per square foot (of course there is a built in safety factor). Assuming a 4' X 1.5' 75 gallon aquarium, that gives you 6 square feet. 6 X 40 = 240 pounds could be placed anywhere long term with no worry whatsoever. Before I forget, you are MUCH better off with a wood stand which spreads the load all over instead of the metal stands which really concentrate the load into four small areas. For weight of the aquarium, the tank weighs around 125 pounds. 70 gallons of water X 8.3 pounds per gallon = 581 pounds, 5 gallons of gravel around 50 pounds, and 20 pounds of rocks totals 776 pounds. The safe permanent load came up as 240 pounds, and you are over three times that, so obviously you can't safely place it in the center of a room! You also will obviously be exceeding the standard design load limits by a considerable amount. In one of the answers, someone mentioned refrigerators. A kitchen is designed to hold extra refrigerator weight with extra joists just as a bathtub has extra joists supporting it. Look at the ceiling in your basement under these areas, and you will see double, and sometimes triple joists. Other rooms are not designed in such a manner as it costs extra money, and is not needed. One of the most important things is to place it perpendicular to the direction of your floor joists. This spreads the load. Ideally, you could place it directly over a bearing wall, or column. It would not at all be a problem then. Usually this is not possible. Next best would be either on an outside wall perpendicular to the floor joists or close to a load bearing wall, also perpendicular to the floor joists. Either of those should allow you to not worry about long term sagging or worse (in my non-professional opinion). Does it make sense? Look on Craigslist if you want to save a few hundred dollars.

I'm sure you'll be fine. Upper floors are usually build with a little more loading in mind for reducing deflection, since your joists have drywall on the undersides, and nobody likes a cracked or squeaky floor upstairs. I built my house with 9 1/2" I-joists on the bottom floor, spanning 10 feet max, and upstairs the floor is 11 7/8" joists spanning 12 feet max. I know the bathtub upstairs with my wife and I in it weighs as much as your fishtank, and we didn't make any considerations for the floor. My wood stove weighs 600 lbs, and the hearth pad weighs 100, plus 50 lbs of hardwood in the fire box... never a problem. I'm sure you'll be fine.

And, your name is accurate.