I fill the buckets with old pond water taken from the top of the pond, reasonably clean, not tap water. You would need to use dechlorinator if you use tap water, i'd rather stick with what they are already in.
I only comptlely drain the pond maybe once every 3 years though. IMO, you need some gunk on the bottom for frogs, tadpoles, and dragonfly larvae to live in. Spring and fall i usually just use a net and scoop SOME of the gunk out of it, not all.