what grade of sand are you using?The new Neros have stronger magnets, I called and verified with AI before I purchased them. They actually hold pretty darn good. I haven't had one problem.
Regarding them being on the back, I was tired of moving them when I cleaned my glass and everything. The only issue I've had with it is it does mess the sand in the front up a bit, but it looks a lot cleaner. I may add a third one to go across the tank from the side.