If you are barely topping off your pond then municipal water is ok. Any hard minerals or chlorine comes in small enough doses that your pond should be able to handle it.
But if you are topping it off with more than 15% of the water then you’ll need a dechlorinator. Or if you forget to take the house out.
Of course, you can avoid these issues by using an automatic filling device (or having a pondless water feature).
Your auto fill system will periodically allow in a little bit of water, just enough to keep the pond full without flooding it with municipal water. The fish and plants will be just fine and your pond will be full.
As for pondless water features, this device can keep a small pool of water at the bottom full and/or help keep the appropriate amount of water cycling through.
You do less work and your pond works better, it sounds like a win-win!
Speaking of work, let’s look what you’d need to do to install this pond auto fill device.