If your ammonia is 0.5 you need to do another water change as soon as possible. You need to keep it from ever getting above 0.25. The black neon may be hardier than a neon but it will still be harmed by ammonia and nitrite even if it doesn't die quickly. If you change 50% if the water, that will only get the ammonia level down to 0.25 and it'll soon get above that. Changing 90% will get it much lower. That way you'll be able to keep it below 0.25 rather than it getting above before you test again. You need to test at least once a day, preferably more often, and do a water change whenever the reading shows above zero even if it's still less than 0.25.
The same applies to nitrite, that too must never be allowed to get to 0.25.
The snail - they do have a trapdoor they can pull shut if the water isn't to their liking but they can't stay like that. Put it back in the tank and make sure the shell opening is downwards. If ever you see it on its back, turn it over. Some fish will nip at the snail's body as it comes out to try and right itself, so it'll go back in a again. They can be stranded on their backs permanently if the fish keep nipping at them, so I always turn them over. If you keep your ammonia and nitrite low, that'll be good for the snail as well as the fish.
The same applies to nitrite, that too must never be allowed to get to 0.25.
The snail - they do have a trapdoor they can pull shut if the water isn't to their liking but they can't stay like that. Put it back in the tank and make sure the shell opening is downwards. If ever you see it on its back, turn it over. Some fish will nip at the snail's body as it comes out to try and right itself, so it'll go back in a again. They can be stranded on their backs permanently if the fish keep nipping at them, so I always turn them over. If you keep your ammonia and nitrite low, that'll be good for the snail as well as the fish.