I've found that the only suction cups that I can get to work are the ones that Marineland sends with their heaters, and the ones Hagen (fluval) sends with their filters. All other heater and filter suction cups seem to lose their suction capability after a while. I've had the same suction cups for my fluvals since I got them almost 2 years ago, and they were used when I got them.
"What, exactly, is the internet? Basically it is a global network exchanging digitized data in such a way that any computer, anywhere, that is equipped with a device called a 'modem', can make a noise like a duck choking on a kazoo." - Dave Barry