water companies will purge or sanitize the lines with a shock treatment. safe for you and me but not the fish. May also be attributed low oxygen levels do to heat, with the 2 combined fish not stand a chance, I personally always used SeaChem safe and always over dosed. up to 5 times recommended is safe for the fish. I have a betta at work and we are on a well that is chlorinated, I dose a 10 gallon for a 50 gallon tank, fish is fine and has been for almost 2 years.