If it's a glass tank with a rim the top won't matter a hill of beans. The tank only sits on the edges. Putting a nice piece of plywood will only appease your own mind. The tank won't be resting on any part except the edges.
I like the look of the 2x6 top but I am a tad concerned about the boards curling up a bit and ruining my perfectly flat top.
What if I put a piece of 1/2 or 3/4 plywood under the 2x6's and glued/screwed them together? That should keep the 2x6's flat shouldn't it?
Or at least keep them from warping any more than they are when I buy them. I can run a sander over the top before staining to achieve a 'flat now' suface but I wonder if it will warp up the first day it doesn't have a full tank on top of it.
This concern about warping is probably just paranoia on my part but I like to cover all the bases before I cut once.