Any detectable ammonia or nitrite is not perfect water parameters.
Do you do a PWC
every two weeks?
How many litres are you changing during your PWC
You should be doing a 10% PWC
every week or 20% every other week.
Are you using RODI
water for top off and mixing salt, or tap water?
The snails most likely were NOT the cause. You were seriously overstocked with the number of fish you had in that small space. A pair of clowns, maybe. A yellow (or any) tang, no way. And definitely not an anemone.
Return the tang and probably the wrasse. What type is it?
You need to check a site that lists minimum tank sizes for fish and adult size that a fish will grow to. Sites like liveaquaria.com are very useful for that information.