What type of snail is in my tank?

I'm assuming they came with the frill.... I have only found 2 in the 2 months ive had it. I seperated them and moved one to another tank. Right now they are small.... maybe half a centimeter in length..... tiny... kind of a spiral shell.... not like a land snail... more cone shaped... brown with maybe some spekles or banding of light brown and some darker brown.... what kind of snail is this? how big does it get? Will it eat my plants? If it stays down to just the one i really have no problem with it in my tank...