They are an oddball size, 2x9 exactly..as most 2x10s I’ve seen are actually 9.25 when measured..however the tank will be located almost immediately on top of the footer. From footer to closest beam is a span of 8 feet. Beam is supported by concrete pads and if I remember 8 inch blocks..or 10s I don’t remember. I can’t believe I didn’t measure what the width on center was apart..I imagine standard building practice of 16 on center..
If I’m correct the one length of the tank will be directly right on top of the footer and the other will be the width of the tank away onto the beams, I’ll have to take a better look to be 100% sure
I hope the subfloor is plywood and not particle board, a lot of cheap houses use it and you would have a real problem if your tank leaked or had an accidental spill and it somehow soaked through. I wish my father-in-law was still alive he was a home builder/carpenter. He was a perfectionist and he could tell you in a second what to do. If you put planks down that would help spread out the weight instead of just the front and back legs of the stand it would help protect against the subfloor squishing to. The other thing you could do is install a support under the joist in the middle (4 foot) to support both joist from underneath that would really help 2x9 are solid joist but with only two it maybe troublesome. You could use cement blocks to build the support.
4 joists sounds much better, Just divide the total weight of the tank, stand and water plus whatever you plan on putting into the tank by 4 to give you the approximate pressure on each joist. If you are correct about the footer and beam that is good.