If you're worried about the structure supporting the tank, you might consider putting plywood, or planks underneath the base of the tank. That way, the weight from the center of the tank would be distributed to the structural members you already have in place. I don't know if this is a simple, or complex solution, but by simply making the base more table-like, the weight coming down in the center can be adequately supported. For what it counts, I think you have enough support, but you just need to make sure that the weight load is getting distributed the most effecient way.
It looks fantabulous! I can't wait to get a home some day, and put something like this in there.