Banded trochus snails

So I went to pick up some regular old astrea snails today from the lfs and ran across some really cool snails that my guy there recommended highly because they are the best algae eaters and they right them selves if they fall on the tops of their shells...and they looked COOL! So I got ten...funny thing...when he put them in the container, some of them were upside down and came all the way out of their shells to push the shell over and go back in...the snails inside are black. Awesome colors and patterns for a snail...here is a picture of one...these snails get pretty big huh? I have a couple shells of small ones but then I have a shell of a really big one I got when I was a kid from Australia...it's white and purple...about the size of a fist. Will these get that big too?

These are ultimate cool too...there are sooooo many cool variations of snail shells!!! Different colors and patterns...are they rare?

So yesterday when I got the snails, I also got some coraline shavings too because my tank has no coraline algae...well the snails had NONE on them...I left the house for a a little bit today and I came back and it was growing on my new snail shells! A couple of the snails also have barnacles on them...will they spread throughout the tank as well?

Ok these snails are amazing workers...they are eating every type of nasty algae there is and leaving nothing behind...brown, green, black, red...no mercy! They look cool too! Very strong from what I see as well and smart...I'm surprised they are not a common snail amongst people's tanks...I guess it would be better to keep it a secret!