I have 2 nerite snails in my 20 gallon shellie tank-they are big guys about 3 years old. Last Saturday I cleaned and rescaped the tank. I also added sand sonce the shellies were digging to the glass. I saw my tiger nerite then and moved him. He seemed a bit sluggish but ok. I have not seen him since! I checked under and behind the stand the hob filters (with a flashlight!), the decorations, the sponge filters, under the rim of the tank, and as a last resort I turned over the shells and sifted through the sand. Nothing! Dont think he died anywhere since today the ammonia is 0, nitrites are 0 and nitrates are under 20. This snail is a big guy and hard to miss, but I worry that I missed him while sifting the sand. Can they burrow out from under 4 or five inches? I have found dead ones ( empty shells) in sand before after they went missing for awhile, but those were all smaller ones that usually recently acquired. The shellies in this tank make hills up to five inches high. I’ve had Burch since March 2018 and am very worried! Is there anywhere else I should look? Any advice is appreciated!