Have a professional inspect your sub floor. If ya don't, make sure you get insurance that covers a broken tank and the water damage it causes. But, you might have problems getting a claim paid if its from a tank that was to much for the floor.
Look for $30,000 in damages or more if the floor fails with the tank full of water. (Don't sleep under it.)
That being said, I have a 75 on a load bearing wall of a 3 story cookie cutter apartment. It's fine so far, but that's useless to you, because some homes can hold 180 gallons, there are stories of 75's falling through the floor in other homes. Each floor is different, even a little damage to the subfloor can weaken it.
If you're feeling lucky then just fill'er up, otherwise have it inspected.