Can you give some details on the tank please?
Freshwater or saltwater?
Size?
What do you have living in there?
What is your normal water change schedule?
Is it an established tank or new? How long has it been set up?
If new, is it cycled? How did you cycle the tank?
What are your water parameters exactly? Both in the tank and from your tap? What are you using to do your tests?
Are you seeing any ill health in your fish and snails to cause you to worry or is it purely some perceived issue resulting from your water testing alone?
Anything you can think of to add may be relevant.
On the face of it, nitrate isnt all that of an issue and get quite high without being an issue for fish health. Its also pretty common to have nitrate in your tap water.
GH again may not be an issue. Maintaining steady water parameters without it swinging about is more important than trying to attain what you might consider ideal. If you bought your fish locally, they are likely bred and raised in similar water to whats coming out of your tap so are well acclimated to it.