crvz,
1) That design is more then acceptable for the size tank you are describing. The overall structure looks well thought out.
2) Since the top frame is only 72" long and you have pieces on both sides of the corner, you only have about 62" between the legs (72" -1.5"*2 - 3.5"*2). Given this span, you should have no need for a center brace.
3) Personally, I don't like wood on concrete. Both materials are porous and since the concrete is right on the ground, it will wick moisture into the end grain of the wood. Consider putting something like tar paper or better yet a small piece of vapor barrier under the legs before you place them. Then again anything that will resist moisture and the pressure of the tank will help it last longer. If there is some form of barrier to moisture, regular lumber would be my recommendation.
scott26,
I would consider looking at the pocket hole jig. The Kreg jig that I have used has a specific stop point on the drill for the thickness of material but the hole always intersects the wood at the same distance from the end of the piece. If you used screws that have a taper under the head, you need to start over as this will split the wood at the bottom of the hole.
RocketEngineer