I was going to suggest looking at things like their kitchen carts, since they seem to be fairly sturdy, but bikinibottom beat me to it.
If side-side/shear(?) reinforcement is a concern, check the area with the modular wooden shelving, I think there was a reinforcing item that was a pair of metal rods, riveted in the center, with holes in each end. Just form into an X that fits the back/side/legs/whatever, and screw it in, should add some stability.
As most people said above, I'd be hesitant to use Ikea (or most other furniture) for an aquarium stand for anything much over 10 gallons or so...