You have good advice above.
Have you got the basics about fish keeping and cycling a tank?
The 1st article below will help you and give you a resource to refer back to when needed. The additional articles are because you have a fish right now, and it might be that the tank is not cycled and the snail is being affected by that.
Do you use a dechlorinator product now, not just tap water?
Do you know about the nitrification cycle?
Do you have a test kit to identify ammonia, nitrite and nitrAte?
How about
TDS,
KH aka Calcium aka Alkalinity?
General Hardness /
GH?
Doing a water change as previously mentioned will continue to help if the snail is suffering from some effect from a toxic water situation.
The main thing is to make sure you do not get more spikes moving the water to dangerous and deadly.
Having a cycled filter will help.
The more times there are unsafe water conditions it stresses and injures the fish or snail until there is a disease or death.
Guide to Starting a Freshwater Aquarium - Aquarium Advice
I just learned about cycling but I already have fish. What now?! - Aquarium Advice
Fish-in Cycling: Step over into the dark side - Aquarium Advice