When I upgraded tanks to a thicker walled aquarium, I asked Julian Sprung of Two Little Fishies to make me a bigger one with a stronger magnet to handle the voracious appetite of my naso, Spock.  A few months later he handed me a prototype to see what Spock could do and it held up nicely.  A year later, he released the MegaVeggieMag, and it's a beauty.  

The inner half still is buoyant and will float if it becomes separated from the outer portion accidentally.  That outer half has a nice neoprene wrap that is soft to the touch encasing a very powerful magnet. It has worked very well on my 400g reef that has 3/4" glass, and the tangs have not been able to pull it apart once. If you have fish that aggressively hit the nori, the MegaVeggieMag is what you want.