Water being used from the tap MUST be treated with some kind of water conditioner...one that removes chlorine, chloramine and amonia. You can also use bottled water as well...they are great but they are way too expensive...especially with over 20 bettas or so. Bottled water don't need to be treated for anything. You can also use distilled water but I don't really recommend it. This is because distilled water have nothing in it...no minerals at all...and bettas need the minerals that are in the water.

Tips:

• Add in a small amount of aquarium salt to help fight bacteria. But don't add too much though...because too much can kill them too.

use a vaccum cleaner such as this to make changing the water much easier.

Now that I'm done explaining all that water temperature lets get to the cleaning part. You should change your bettas water 2-3 times a week if you are housing them in jars. Change their water every week if you are housing them in tanks. If you are housing them in a community tanks just do partial water changes every week...about 25% of the water. Then do a full water change every month or so. 

When you change the water use a fine mesh net ( such as a brine shrimp net) to catch your bettas. You can use the green nets but they might damage your betta's fins. Then place them in another tank or jar and change the water. Just clean their tank or jar....use some salt, it helps make cleaning easier. I should also tell you, although I've said it before, don't use any soap. Soap is poisonous to fishes so it will probably kill them.

a brine shrimp net such as this can be used and is best recommended.