How about these... http://www.kjmagnetics.com/proddetai...od=BX084PC-BLK

jason - you'll want the N52 grade coated magnets. Though i believe that site states those should be used for aquariums (possibly due to the coating being super thin to maintain the magnets super strength?)

dyi, If I needed, I'd be all over this.

KJ magnetics also tells you how to rubberize the magnets. This would work well in the aquarium and not corrode the metal.
Although if you do this it will have less strength as the magnets will be further from each other.
I wonder how much magnetic force you would lose?

http://www.kjmagnetics.com/rubbercoat.asp

I wrote KJ and they responded right away. kudos to them
Here are two links which would help us make frag shelves. The first one with a max load table.
Second with a formula

http://www.kjmagnetics.com/blog.asp?p=aquarium

http://www.kjmagnetics.com/pdfs/aquariumshelf.pdf