Are the joists going underneath the tank and if so, how many... Or are the joist running horizontal with the tank (this isn't good)
It looks like you have a silent floor setup... If so, I have the same, mine has laminated plywood 2X4's with the middle glued wood... this is suppose to be some of the strongest for supports... When I setup my tank, I made sure to have at least 3 of these running underneath, I might even have a fourth, I am really not sure there. Anyways, it has been over 1.5 years and it hasn't budged. I keep a level with my tank just to test and make sure, and it hasn't moved, and I am on the 2nd level of my house. My tank is a 180G DT with a 105G refuge, so I have quite a bit of weight there.
Mine is on 2 load bearing walls, like in your diagram.
If your house is the same as mine, I wouldn't worry, as long as you have the joist running under the tank.
I hope this helps