Hey there,
It is difficult to determine which snails you have from the pictures provided. Typically the snail needs to be taken out of the water and positioned carefully to have a chance at an ID beyond a common name.
Your first picture could easily be a Nassarius snail (usually good), a Columbellid snail (usually good) or a Whelk (usually bad). You can see some example identification pictures in this thread, post #5:
Columbellid sp. (Easy) - Project DIBS Forums
The most important picture is the first one showing the shell opening with the snail completely withdrawn into the shell.
Your second picture could be a cerith snail, but it could also be numerous other Genus of snails, some of them good, some of them bad. I cannot tell from that picture, if you could take additional pictures I could try to narrow it down some or point you to someone who might be able to help.
Brian
