If the way you have it in your diagram is correct you would not have you tank where the green or blue areas are as the whole weight of the tank would be on 1 joist. Joists run the opposite way to your floor boards, so as you look at your diagram your joists will run from top to bottom, putting a tank in the corner wont be supported by 2 load barring walls as all the weight is on the joists which only go into oppositing walls not connecting ones. Its best to put the tank on an outside wall if possible not room dividers as them walls are usually thinner, as mentioned before take any feet that are on bottom of your stand so weight is distributed accross the base of the stand instead of 4 pounts. Check your joists for there condition, any that have got damp or have an infestation will heavily weaken them andwhat is the dimensions of your joists? I had a 200l on 3 6x2 joists and have a 400l spread over 4 8x2 joists, the 400l is also screwed to the wall to take some weight of the floor, both upstairs. I would say yours should be fine if your joists are in fine condition but if you are really going to worry about this then get a structural engineer in.
