sounds like it would work... i can't say for sure... although i think your on the right track by not using what comes with the racks... they say they can hold like 1000 lbs, but i would trust it... what size tanks are you thinking about using on the shelves? if you give as much info as possible, others might be able to help out a little more...
3- 36" wide shelfs with 3-40G Breeders
2- 48" wide shelfs with 3-33G Longs or 3-50G Breeders
4- 30" wide shelfs with various grow out tanks 10G and 20G longs
After i made this thread i found a guy who actually put (4) 40G Breeders on one shelf and it worked so i assume mine should work out as well since i think those 4 tanks is pushing the limits though IMO
The 2x4's i plan on running vertical instead horizontal i think the short pieces will hold up better then the long pieces which could start to bow after some time
I've not used the prefabbed style shelfs for racks. I thought about it but was a little concerned about a couple of issues. One is that the load they say they're capable of handling is more about at the 'joints' and less about how the center of the span will deflect under that load. Just heavy boxes on a shelf--who cares if the middle sags some? A glass tank ...........
I see, in the pic of the rack you posted, some cinder block supports on the floor under the lowest rung of tanks--also note that the same size tank on the un-additionally-supported upper rung is less than 1/2 full. All things to consider