Your stand design is stout to say the least. One thought occurs to me though. At 675 or so gallons this could weigh somewhere in the neighborhood of 6000 Lbs combined. That's 1000 Lbs per leg X6. Are you on a concrete slab or is your subfloor/foundation beefed up to support this ?
dawnmarie has a point. You want to have the perimeter of the stand resting directly on the floor to distribute the weight. The more linear inches that contact the ground the better.
I live in a 230 year old post and beam house with a ledge floored basement. First thing about that comes with the age of the house is that the floors are not anywhere near level or flat so even if I tried to have the whole perimiter on the floor it just wouldn't work. The second thing about the age of the house is that framing is just about bomb proof . The rear legs will be directly on top of a 10X10 solid chestnut beam which spans about 14feet and and rests with one end on the foundation and the other on a 10X10 post . The front is however a different story. They currently sit on a dimensional 3X10 floor joist. This I will be trippled with hemlock planks cut on a neighbors sawmill and a 6X6 PT post will be directly under each of the end legs.
So I've got a little bit of a change in plans. My brother works in a local lumberyard and when I asked him to get me a price on 6X6 kd he came up with somthing better. He found 12 9" I- joists that were slightly discolored and out of inventory. After talking with the yard boss my cost was exactly $0. This was a deal I could not refuse. So now come my questions: If I run 4 of them from end to end 9" on center to carry the tank would they be able to handle the load of about three tons of water without flexing? If so any suggestions on how to secure them to the legs without compromising the stability of the tank?