First what type of snail are they? I've found turbos to be the only ones to knock stuff over really. My ceriths and astreas dont even touch anything besides rock, sand, and glass.
Second, 9 is probably 5 too many for just a 9g tank....that is, depending on what type. If personally always have a lot more snails than people suggest. But they dont die due to lack of food (i got lots of crap for em' to eat and i prolly overfeed lol) and you cant say they're "too crowded" cuz if you go to any area with snails, you'll see them cramped in together and all over each other.
But what type are they? The only snails i'd keep that high of a ratio (1 per gallon) is astreas and/or ceriths. Even then, unless you have an overabundance of either food or alga, dont get that many.
