As Dr Rob said definantly go for the largest magnet you can get, too small and they just dont have any real suction power. The one I have has a loop on one end of each magnet that you can tie some string to incase the suction fails then you can hail the wayward one back up. But I almost always find the string ended up getting in my way.
Now days I mostly rely on snails, bristle nose catfish and shrimp to take care of my glass, which I just as well because my big freebie tank is so deep if I want to plant anything in it when its full I not only need a step ladder but snorkling gear too.
Now days I mostly rely on snails, bristle nose catfish and shrimp to take care of my glass, which I just as well because my big freebie tank is so deep if I want to plant anything in it when its full I not only need a step ladder but snorkling gear too.