Here's a simple answer: it's not.
Your existing beam supports are less than adequate (to put it politely). Leaving aside the issue of beam and joist sizing, each beam support should have a proper post and a footing sized based on the tributary area, the 50 psf total floor load jadnashua mentioned, and the soil bearing capacity. The soil bearing capacity depends on the soil type, but in the absence of a soils report is often taken as 1500 psf as a reasonable minimum (at least in much of the US, soil types could be different where you are.)
For example, consider a single story building that is 20 feet wide with continuous foundation walls on the perimeter, and a girder down the middle supporting 20' floor joists only (no ceiling or roof load). Then the girder tributary width is 10' (the middle 10' of the 20' span). So if the girder has posts every 5', each post carries 5' x 10' x 50 psf = 2500 lbs. With a 1500 psf bearing capacity, that post should have a footing of at least 1.66 square feet. [And where issues of frost do not require a greater depth, should be at least 12" deep bearing on undisturbed soil.]
Of course, that just covers the usual loads, the tank requires its own supports. You mentioned that the footprint is 35" x 58". At a minimum you'd want a properly sized beam 58" long centered on the footprint, with two properly sized posts and two footings. Since the tank weight is 5000 lbs, if the 1500 psf minimum soil capacity is applicable to your area, each footing would need to be at least 1.66 square feet.
Cheers, Wayne