Suction Cups Won't Stick To Inside Walls Of Tank

So, I've never had a problem with getting those black rubber suction cups to stick to the inside of the tank before, but over the past month, they keep coming unstuck and floating off around the tank.

I've checked old threads here about the cups sometimes getting stiff, but the cups don't seem stiff to me. I had to buy a new thermometer about six weeks back, and that came brand new with a new cup and that's worse than some of the three-year old cups on my filter, and it feels very flexible to the touch.

I saw on another thread about giving the tank a really good clean, and also about making the seal between a dry bit of glass and a dry cup. Well, this weekend I've given the tank a serious scrubbing in the areas where the filter and thermometer sit, using an old plastic scraper. This hasn't helped either.

I haven't moved the tank (or house), or changed anything I use in almost nine months, either. It's the same water, with the same de-chlorinator and so on, so I can't think of anything there that would cause it.

Has anyone got any ideas on how to fix it? The filter's currently being propped up in place by two of four working cups and supported by a plastic plant, while I've given up on the thermometer and left it to float around the tank as it wants.

I had to buy a new thermometer about six weeks back, and that came brand new with a new cup and that's worse than some of the three-year old cups on my filter, and it feels very flexible to the touch.

I agree - I've never succeeded in managing to get the thermometer to stay stuck to the glass. I've ended up having to dangle it from a fish feeding ring (that I use to contain floating plants) until I check its temperature, then remove it.

Have you tried soaking the suction cups in hot water? Sometimes that helps.

I tried this, but it didn't seem to make any difference. The cups felt more flexible, but the moment I put them back into my tank, they cooled down and firmed up again.

I'll just give up and order some more suction cups then, thanks!

Thanks fcmf for the ideas. I've found the right suction cups for my filter online and they're cheaper than the magnets, so I'll give them a go. If it's just that my last set are now old and decrepit then hopefully this will work, but if not, then I'll give the magnets a try instead.