I can likely answer your question regarding overbuilt on the stand. When using the calculator listed below, I selected "Spruce / Pine / Fir" as it has the lowest rating. I also went with Grade 1 instead of Select Structural (again, a lower rating). I then adjusted the unbalanced height for X and Y from 8.5' to 2.5' (basically 30"). Now, If you look on the right of the screen for a 4"x4" column you will see that it has a maximum load rating of 13,952 pounds. That is for 1 column. You have 6 of those columns in your build. Now, your build does not necessarily put those columns directly under the corners of the tank, but, you have a pair of 2x6 boards set under each corner. Assuming they are attached to each other well enough, each corner without the 4x4' should support 10,553 pounds.
As you can see, in a vertical fashion you have definitely over built your stand. The one thing I stress to others when building a stand is the horizontal movement of the stand. In other words, you are chasing a fish around with a net and getting the water sloshing side to side. Lets say you can actually get 50% of your water volume to impart a horizontal load on that stand. Assuming 220 gallons times 8 pounds per gallon then divided by 2 that would be 880 pounds of side load on your stand. This is my biggest concern with the PetSmart / PetCo 3/4" particle board stands yet I don't think I have ever heard of one of those POS collapsing.
So, with all that said and looking at your construction techniques, I would say your stand will definitely NOT collapse under the estimated 1,825 pound load you might put on it (1,760 maximum pounds of water along with substraight, etc).
As for what others have mentioned regarding stress risers from the horizontal mounting surface for the tank, basically it is all good advice! (FYI, given the pair of 2x6's on each corner and the single 2x6 in the middle of the front and back, the calculator listed below shows a maximum vertical capacity of 53,674 pounds.)(If you want a good chuckle, click on my 75 gallon build thread and look at how I did my stand - BEFORE I educated myself with the vertical capacity of a wood column, LOL).
Capacity of a wood column