Yes, you really should support the center of the tank. The lumber is not going to break, but it is going to bow. Do you really want your tank to bow? Regardless of whether it will break or not there just a right way to do things.
I'm sure the wood can hold the load about 1700lbs. However they will probaly bend a little in the middle. If you put some more cinder blocks in the middle it would streghten things up a bit.
What would really help is if you turned the 4x6's on end they will be a lot less likly to bend, and some ply wood on top would help with the narrower size of the beam supportting the tank. If you got a 1/2" sheet of plywood cut in half to could put both pieces under and you would be good to go!
Quote from Truvuaquariums regarding acrylic: "Proper stand selection: Acrylic aquariums require a "solid support top" stand. In the event you can not locate a solid support top stand use of plywood or equivalent will suffice."
You might also throw another board and a piece of plywood in the middle as well. It doesn't have to do with holding the weight, but with supporting the acrylic. Acrylic will bow and sag in the middle without support. This could potentially cause a problem in the future.
Cool thanks for the tips. I will drain the water and put wood underneath the tank. As for putting cinder blocks in the middle, I don't think ill be doing that bc I have no room underneath. My garage is small so the bottom of the tank is storage space for 2 of my rims.. ill update once I start putting stuff in the tank