"Just buy the largest one from the get go - the smaller ones don't have the force or surface area for the recommended tank size (wall thickness). Still have to get in there once in a while with a scraper to get the rounded corners and down along the gravel line. Be careful not to go round the corners with snails on the "glass" - if you happen to trap one under the cleaner, it will gouge your acrylic (trust me). It's the newborn snails that will get you - the big ones you can see and avoid. And don't go too near the gravel line - I use natural gravel and there can be fine particles that are slightly magnetic, which will get under the cleaner and also scratch the acrylic."
