This Old Spouse
TOTM Winner May 2013
Check that snail with a little poke to see if it moves. If it doesn't and it's not upright, it's likely dead, and that will cause an ammonia, therefore nitrite spike. They're really nasty when they die.
Also, without rereading this whole post, have you checked the levels of the tap water you're using?
Also, without rereading this whole post, have you checked the levels of the tap water you're using?