TDS meter for the tap water. Your pH is surprisingly low. What WarDog says actually made a lot of sense, your over consuming bioload from your snails may be a culprit. I had this actually happen in a planted tank with my fish dying off. I had a pond snail break out, and it got so overwhelming I couldn't keep up anymore (damn hermaphroditic snails) I found my tank went through another cycle just based off of the amount of bio load I had in the tank. I had about 70 fish in a 55g, with doubled filtration. Once my fish started dying off, I found that too much bioload was the culprit.
I ended up taking my entire tank down, cut all the leaves off, changed my substrate and replanted and moved my fish back into the tank and all was well after that - and I eventually, after several years - got rid of my snail problem. (Thanks to clown loaches, actually..)
Not saying that this is a direct cause, but it's a acute possibility.
I know you're not new to the hobby by any means, however; try perusing this forum thread:
http://www.thereeftank.com/forums/f6...ed-160389.html - it might answer some questions you need answering.