thank you.Ed Seeley said:Leaving water for a while may help to dissipate chlorine but it won't do much for chloramine. If you have this in your water supply you may well want to use a dechlorinator to ensure it is removed and the chlorine and ammonia is locked up so it cannot harm your fish.
zig said:Add the appropriate amount of prime (usually doseing for the whole tank volume not just the changed volume) to the existing water then add the freshwater to the tank. This is how I would do it if doing a 50% water change.