Here's what I did for my 200. Adjust measurements as needed.
My 200 is 6 foot by 2 foot.
16 cinder blocks.
2 1/2" thick 6x2 paint grade shelving boards.
The paint grade shelving just happened to be the cheapest thing I saw when I was at the hardware store that day, anything sturdy should work fine. Just no MDF.
I put down 8 cinder blocks on their side holes facing out, one board on top. Another 8 cinder blocks on the board (still holes facing out), then the 2nd board then the tank. I haven't had a chance to yet but will be spray painting the entire thing. Cinder block openings are facing out so the kids can get goofy little decorations to sit in them. Even though I know it's not going anywhere, I wouldn't put anything I valued under that much weight.
Other options include getting a third board for the bottom so it's board, bricks, board, brick, board. More symetrical, and I would have done that had I thought of it ahead of time.
Also, with the 1/2" boards, you can always make a nice finished front and screw/nail it into the boards depending on the room.