The problem with a shelving unit of that type is that it is extremely strong against vertical forces, but pushes from the side, especially when it's under a heavy load, can cause the entire unit to collapse because there is virtually no reinforcement against that type of force (it's called racking.)
The reinforcement is easy though, you'd just buy a longish strip of steel (or a perforated strip of steel, don't know what's available to you) and mount it diagonally across the back of the shelves by drilling holes in the appropriate places and bolting it onto the frame via the holes in the legs. It wouldn't hurt to do the same on the sides as well, but the back is the real concern, as the larger rectangle shapes are far easier to distort into parallelograms. The point is to convert those rectangular shapes into triangles that can't be distorted without tearing the shelving material apart. Multiple strips in multiple directions (a giant X, for instance, or a sideways V shape) would be even sturdier, but isn't necessary.
I've seen people using that type of shelving for tanks before (or pictures of it, anyway.) I've built my own from lumber, and for 3 smaller tanks of the size you're wanting, I use a very sturdy bookshelf.