PH really low, add some baking soda to the tank. It will increase the PH. What change in the water are we talking about? If anything, a snail dying in a tank will increase ammonia. Taking it out will correct that. Brown spots sounds like algae too, but at such low PH is not normal I would say. Snails have a life span, so how old was it? Did you have it long? Could have been the low PH. Not an expert, but I think snails need high PH. Like really high PH, around 8 or so. Yellow on PH readings is 6 or so, which is very low. I would water change about 50%, and fix the PH with baking soda (which works really good). Change in water? Did the ammonia level rise? Nitrite rise? Those could have just been the snail in the water when it passed. Taking it out would just fix that. I just recently lost a bunch of nerite snails myself, but my tank was still cycling and I think thats what did them in.