It's possible the .25 ammonia reading is from the snails dieing and not the original killer in your tank.
Find all the dead animals and remove them, then do a large water change, I'd say 40-50% stretched over 3 days. Then retest your water parameters, and if you still have ammonia at that point make a check list.
1)any new Live rock? (might have decaying items)
2)have you ever added any chemicals? (ICH meds, ect.)
3)stray voltage? (bad heater)
4)take water to your LFS for testing
5)do any of your tank levels fluctuate? (temp, PH, ect.)