Licensed carpenter, no. I did work as a framer for a little more than five years. Loved the work. Hated the splinters! When my boss told me to "Put some more wood under that", I did. One good infection from salt treated lumber and a close call while installing trusses, I chose to move on.
Please don't get me wrong. I think this is one of the coolest things i have seen someone do to their home. And the enjoyment you will get from this tank is far beyond my comprehension. Please don't take my input as an attack.
My point is that inch pounds and foot pounds are not legitimate arguments when you are talking about a 24" span. A simple suggestion would be to fill those holes with more stacked 2x6's so that your foundation supports the full load. I've seen 10" footers move with time. (Granted, in Delaware we are floating everything in very sandy soil close to the water table) It's just insurance. The cost in wood would be a drop in the bucket compared to what you already have. 20 tons is nothing to turn your nose up to.
Best of luck. Keep it real.