I read different things about suction cups not staying stuck to the inside of the aquariums the way that they are supposed to and I have had issues with this myself. I think that I have found the cure to the problem. I just recently tried this and it seems to be working. I just cleaned my Plant Propagation Tank and removed my Sponge Bio Filters to clean them and replaced them with ones that were previously cleaned. About an hour later one of the filters was floating on top of the tank, so I pushed it back on and this kept happening so I put a new set of suction cups on it and it still floated and the other one started floating too. After being frustrated about it, I thought, I know the glass is clean, and the new suction cups are clean. I put a real thin layer of Petroleum Jelly on the cups and they are still stuck. I have done this on all of my suction cups since and it works. The little bit of film of jelly is not enough to harm the fish even if petroleum jelly would harm them, which I don't think it would.