Yah, I"m pretty sure it's 3/8, and the box said 3/8 was the max, but they clearly meant 3/8 was the max. That's just for the rack to hold itself up.

Unfortunately (or fortunately in the grander scheme of things) I've got Eurobracing, so hanging arms won't work.

The other times it fell, it just sort of slid down the glass so that the heavier corner was resting on the sand, and it was doing it slowly while I wasn't in the room. This time I was sitting at my computer (where the frag rack is in my peripheral vision a few feet away) literally typing out a post for this thread when one of the magnets clattered to the floor - I look over just in time to see the frags all tumble out on to the sand. I have a new coral beauty angelfish (keeping my fingers crossed tight that this guy isn't an SPS connoisseur) that has been all over the film algae that's starting on the frag rack, I think he may be the proverbial straw. Over and over and over again.

Do you think I could get away with only putting the neodymium magnets on the outside of the glass, or would I need to replace the ones that are on the water side as well? If just the outside, is the plasti-dip still required, or will the scratch the glass without it?

For now, use the mag float on the top lip, not underneath as support. Sandwich that top piece, it works well with a full rack. You're lucky, I had a large torch under my rack, so when it fell, I lost frags to the torch each time.

good idea, I just did that. It had never fallen completely off the glass before, just slid down, so I thought having it rest on the top of the mag float would be enough. WRONG!

The sandwiched effect seems much more stable