Is it safe to put a 60 gal aquarium on the middle floor of the house? In theory it would weigh about 720lbs with water.?

I personally never recommend fish tanks larger than about 40 gallons on anything except the first floor UNLESS you have someone such as a contractor come in and tell you if it is okay or not. It might work if you could find a spot with a support beam, but as said, you should still have a professional come in and tell you if it's okay or not. It's better to pay for someone to come and check it out than just do it and possibly have 720 pounds of water crash through your floor one day.

A 60 should be fine, better if next to a wall rather than in the middle of a floor.

Water weight is under 500lbs, plus 150 in tank weight and rocks .

If your floor could take 4 big guys, it can take the tank. If it can't take that much weight, you need to move.

I would not suggest it, but here is what you have to consider:

Strength of the floor, and support(if any).

To find the strength you have to find out exactly what it is made out of, then you have to figure out if the floor can take it.

I would suggest putting it near a supportive wall, or asking a professional, because you can make a giant hole in the floor.