Eventually it gets to the point where the water in a tadpole container needs to be changed.  For the replacement water I use well water, as the tap water contains chlorine.  I empty about half of the water from the container and pour it into another container.  The pouring into another container is a safety measure in case any tadpoles are accidentally poured out.  I let the replacement water and the container sit in the same environment for some time to allow for the temperatures to equalize.  Then of course I slowly pour the replacement water in.

Going a bit off-topic, I've noticed that the tadpoles have been doing fine without me blowing bubbles into the water to dissolve some oxygen.