+1 I don't see any possible way for the magnet to crack a tank. It just doesn't put that kind of stress on the glass. It's sorta like how people blame snails and starfish for killing their fish when they see it being eaten by them when they weren't the cause of the death - only cleaning up the mess as nature intended.I think I'd have to agree with Hydor on this one. Seems more likely that there was an imperfection in the glass and you had the bad luck to put the magnet right on it. See if you can find a glazier willing to look at it. I had a window in my apartment crack and the glass maker was able to tell immediately from the nature of the crack that it was due to the building shifting and putting weight on one corner. Maybe one could pinpoint the cause of the crack for you.
I've used those kind of PHs for years as have many others with no ill effects. Lots of aquarium equipment is attached via magnets. I see the desire to place blame somewhere and have the cost refunded but it's certainly not Hydors fault. The tank manufacturer will place the blame on the weather if they can. Their job is to make money and they can't do that by replacing every damaged tank and livestock without a fight. Few replace livestock even when its their defect at fault.