Its either a bad batch of Dechlor (prime) or the local water company is adding stronger amounts of chemicals safe for us but not safe for fish when late summer due to low water values and or high flood values this is more common with water companies this time a year..
Also i do this every time check the date on the bottle of whatever you buy if its more than 3 yrs old i wouldn't use it and some don't even use it 2 years past..