Thing is you don't have to cook the whole batch of water. I used to do 10g waterchanges and only had to boil a pot full of water, a couple of liters (not even boil it just until it was steaming) then mix it into the main batch of water, make sure it was the right temp and away we go. I did this when I was getting water from the grocery store or had to do "emergency/unplanned" waterchanges. I never saw any detremental effects but then again my tank was never all that great looking back then either
