Ah... the floating shelf.
Have you considered using black steel pipe for stand columns and tees for corners? I have a 5.5 gal tank and 4 plant pots on top. Pretty strong. I mounted it to the stock stand with bolts and steel plate from within the stand.
All parts can be had from your local Home Despot or bLowes, and it won't break your bank.
2x 5' black steel pipe
2x 1' black steel pipe
2x steel caps (for ends of the short pipe)
4x bolts + nuts + washers
2x steel corners to reinforce the bottom (just in case)
The pipes come already threaded, so assembly is real easy - just screw the pipes into the tees, and - presto! - you've got an L-bracket.
Tools needed: drill and drill bits, a socket wrench to tighten the bolts (although good pliers should do in a pinch)
I sprayed the assembled pipes with black paint to match the stand.
Oh, and the shelf itself is just 3/4" plywood with some edge made out of poplar plank, all painted black.
There! That should do it.
Hope that helps.
29g eco, DIY up to 81W PC (55W used), Rena Filstar XP2 Filter
Corkscrew Vals, 2 angels, 4 corydoras aeneus, Ramshorn snails, MTS
5 gal RCS tank with sand bottom, java moss, small internal filter, MTS, Ramshorn snails.