Okay-- just a quick one ---
a 2000 lb stand + a 1700 lb tank + 1200 gallons of salt @ 8 lbs per (roughly) comes out to 13,300 lbs.
Divide that by your 6 legs and you'll get around 2, 216 lbs per leg
say your legs are 8x8 inches with a plate on the bottom of 64 sq inches you get 34lbs per sq inch on each leg.
If you don't add in the rocks, sand and the stand (which could be supported by the ply frame resting on the floor) you've got a lot of weight per sq inch. Still, if the 4" slab is well reinforced (steel) and it isn't resting on soft substrate, you should be okay.
If I were actually an engineer and hadn't just slept at a Holiday Inn, you could take that to the bank.