Originally Posted by aaamborghini
It would be best if you treated the new water the night before changing it and let it sit at room temperature.
Originally Posted by Beall
You should treat the water 24hrs before you do a water change. Gives the conditioner time to do its job and will allow the water to warm to room temperature.
What you two have said on how to do the water change is all perfectly correct, apart from this bit.
Firstly, room temperature is too cold - you want the water to be as close to tank temperature as possible, which is easier when you are getting from the tank. Letting it sit is just going to cool it down.
Secondly, water conditioners work straight away, or within a few minutes at worst. Time was people didn't have dechlorinator and let the water sit out for 24 hours to allow it to evaporate, but we don't need to do that anymore (nor is it advisable, as chloramine doesn't evaporate during that period).
So, when you get the water from the tap into your bucket, try to get it as close to 80F as possible, then add the water conditioner, then go about acclimatising your betta as Aaamborghini and Beall said. :)