I can't keep snails and Blue leg schrimp

So, my tank is around 8 months old, 16 gallon nano. 2 Firefishes are doing great, one Yellow clown goby too. Peppermint schrimp no problem, 3 Nassarius snails all happy BUT any other snail (Turbo, Cerith, Counch,..) or Blue leg schrimps are dead. They are ok for a month or so, but then...dead.



Any idea why?