I have that shelf!! I got mine at target! I'm doing the exact same thing you are! Its very strong. I sat on each one of the shelves as I put them on to make sure they would hold my tanks. It does great!
I've been meaning to put up pics of my fish shelf but I haven't gotten around to cleaning my room. I'll have to put pics up tomorrow.
I don't use the top shelf of mine though (except for storage). I'm 5'2" and I still need a stool to get to the tanks on the upper shelf LOL
I'm also 5'2" that's why I like the size of the shelf I'm ordering...only 54" high...right about to my shoulder so I can put smaller(2.5g) tanks on top....but 72" wide it covers alot of wall and my 20g long will fit too. Been searching for this size for awhile.