+1 The ammonia may be visible on the test kit but it should be neutralised from causing harm; however, if there has been some sort of chemical instability with the water conditioner, that neutralisation process might be inhibited. That would be an awful lot of chlorine to cause that much of an ammonia spike though...a bit of a long shot but it might pay to get some fresh stuff and see how that goes.
I also wondered about the substrate, and about what the council has put in the water recently. That snail migration is a bit alarming.