Check your Astrea snails (and any clams) for parasitic Paramidellid snails. If you don't find any of these tiny snails on your Astreas (or clams), then you likely have Rissoid snails. They tend to come out mostly at night. I have a lot of them in my tank. Paramidellid snails are parasitic, and Rissoids are algae grazers. The most obvious physical difference between the Paramidellid and Rissoid is in the tip of the shell, and it is hard to describe, so do a search on Rissoids, and then get out your magnifying glass.
