In response to your ammonia dilemma, I have a few suggestions/questions that might help out. First of all, your tank is still relatively young (approximately 4 months old) so your tank might still be undergoing some mini cycles as a way of coping with the larger number of fish you have. Also, you did not specify your water change schedule or the type of water you use to perform your water changes. In order to have a thriving saltwater tank, it is imperative that you use RODI
water when performing water changes. Likewise, I would highly suggest performing water changes a bit more consecutively until you manage to lower the level of ammonia in your system. As for your snails, they might be dying since your tank is newer and there isn't enough algae for them to eat. Most likely the ammonia is too much stress on them, since they cannot usually tolerate any ammonia in their water.
Some things that could possibly help:
-get your water tested by your local fish store, since your test kit might be harder to read or not as accurate as there's is.
-try feeding your fish less (don't overfeed) so that there are less organics in the water that might break down into ammonia.
-try ammonia reducing products like Zeovit if you are interested in achieving a low nutrient system.