Pyramid snails can hitch hike in on clams an snails. A visual inspection of ALL surfaces of the clam's shell, using a microscope to see the babies and eggs, should be done prior adding it to the DT. Some people do a freshwater dip.
If you see the snails, they can usually be removed manually, but inspect very close to make sure there are no eggs. I used a scalpel and tooth brush to remove them.