They are not suitable for some tank designs. They hold pretty tight on one of my tanks and are pretty much useless on 3 others. I would recommend buying the small size even if you have a large tank as the large size is too loose and doesn't close the cap tightly. They are flexible so the smaller sizes should fit most tanks. However, in 9/10 cases they don't work at all. The only reason I repurchased them was because I got them as an optional item for less than $3. I use bungee cords wrapped around the tank with the ends tied together and this gives the lid a very tight fit on the tank.
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