You must understand that even low levels of chlorine are and instant hurt to your fish if they come in contact with chlorinated water. Even for a short time! This means you MUST dechlorinate any water you add to a pond or tank that has fish in it BEFORE YOU ADD THAT WATER! The best and safest way to ad chlorinated water is to use a know gallonage tub, barrel or whatever filled with the chlorinated water. Then you know the true gallons you are dealing with, and now you can safely dechlorinate it with a dechlorinator. Once you have mixed the water and dechlorinator thoroughly only then is it safe to add to the pond or system.
If you have chlorinated water and an auto fill on your pond I would highly recommend you disconnect and discontinue use of that auto fill. If you don't you will one day pay the ultimate price and end up killing all of your fish!
This is a high quality and accurate chlorine test kit, and while it is somewhat costly, it is a must have if you have chlorinated water as source water for your fish, AKA city/Tap water.
You need to know how much chlorine is in the water to properly dechlorinate it and make it safe for the fish. As well chlorine levels in tap water can change seasonally so you should test the level of chlorine each time you are adding new water to the pond or tank.