Super braces! That was kind of my thinking too, though I hadn't thought of the pegs, I can see how those would help.
I think I'm going to change my plan, save my pennies, and go with pillars of concrete blocks. It can be done for about $25 if I buy new materials from the retail store. Less if I can buy someone's leftovers.
Alternatively, cement in the ground 4 hardwood posts, then use the hardwood supports from the pallets to form the frame at the top of the posts to hold the tank. Cost you about the same amount but wont go anywhere! Will still need the corner braces however to stop lateral movement. Use quick set cement if you want to complete in a single day.