Hi, Diatoms...it happens to all new tanks, as a rule it goes away on it's own. You need lots more than 4 snails, and not all snails like the same thing. In a 30g tank I had 65 snails of different types. In my 56g I have over 100 and I lost count on how many are in the 90g
Turbos
Nassarius
Cerith
Margarita
Astera
Make sure you have plenty of circulation too, the wave is the life of the ocean.