Well if I'm going to be cleaning the filters the water would be from the tank itself, usually I'll siphon out water into the bucket then clear out the sponges. Then I'll dump out the "bad" water, usually into some plants or something it's awesome fertilizer, and refill with tap water. I live in an awesome place for aquariums, the tap water here is slightly hard and there are no harmful chemicals in it that I'm aware of, so I usually just match the temp of the tank and refill it directly.