If you shim the outer edges of a single piece of plywood, you will end up with a tank that is only supported on the left and right edges by a bowed piece of plywood....If it were me, I'd feel safer just putting it on the floor. The carpet and padding will make a nice flat surface for it to sit on. Just make sure it is not in a high traffic area.
Just to be sure, get a long 1/4" or smaller masonary drill bit and drill down to see if you hit concrete past the bricks. Unless you build it yourself you never really know.
Then get a bearskin rug and some brandy and your gal!
^^^ good idea, wanna make sure they didn't do it cheaply and frame it will plywood and build on that. If it seems pretty sound if you jump on it, I would say they used concrete.
one thing on styro foam get the soft kind. the pink stuff used for insualation doesn't compress so it wouldnt fill the voids in the brick. I damaged the mess out of my tank making that assumption.
That isn't a lot of weight, it won't be a problem. It can't be hollow without having support inside, the bricks couldn't have been layed up. If its on a slab it would have been more work to make it hollow than to just lay up the whole area with block/brick so its not going anywhere.