What kind of snail is this?

[BBoley24]

I recognized today that these little things are popping up everywhere. Are they worthless? Harmful? Good?

 

[graveyardworm]

Looks like colonista, I have alot, and believe they may be breeding in my tank.

http://reefcentral.com/forums/showthread.php?s=&threadid=977412

 

[ubernano]

they're harmless little snails, and yes they multiply like crazy, they get annoying after there's like 50 in youre tank. really hard to get rid of.

i know big stores like petco / petsmart have them in all they're tanks, and they occasionally sneak in your bag of fish.