It is highly unlikely your nitrates killed your snails. Your specific gravity is a major concern for sure. What are your other parameters? What is the temperature? Nitrates would not spike that high, that fast, resulting in trouble with your snails. How many snails are involved here?
My temp is between 78 and 80 depending on when the lights are on, and my trates at the last check were between 0 and 5 ppm, and were talking about ten snails. i turned them over last night before went to bed and they still aren't looking good. too much of there body exposed. and none of the crabs got any of them last night. really confused, not sure what went wrong. everyone else is doing just fine
Do you dose the tank with anything? I am really not willing to abandon the copper issue yet if this happened suddenly and is impacting all your snails at once. Different inverts may have different tolerances.
Yes, but it is unusual to me that they would ALL die basically at once. There is individual variation - even in terms of salinity tolerance and such. If ALL your snails are dying at once, I would look to some sort of outside cause, personally. Starvation is always somewhat of an option.