I'm putting a standard 6' long 125 gallon tank on the second floor of my bi-level home. It has to go parallel to the joist. The good news is it will be up against the wall and the center of the tank will be running directly over the steel I beam of the floor. The joists are laid out so the length of the tank will be supported on both sides by a joist underneath it. Basically the support structure is 2 2x10s running underneath the length of the tank for less than 3', then the I beam, then 2 other 2x10s running underneath the length of the tank for less than 3'. No other serious amount of weight is resting on the joists supporting the tank. I figure the proximity of the tank to the steel I beam should be sufficient to support 1,200- 1,500 pounds but I'd like to get some other opinions.