Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
When I am doing a larger water change, such as a continuous drain and fill I dose dechlor into the incoming stream, measured to match the amount of water coming in. So every 5 minutes I might dose for 20 gallons. This way no matter how much I remove (siphoned out to the garden) the incoming water has some chance of being treated.
At the end, when I am no longer draining I will give it one last dose, equal to about half the tank, just in case I was not quite on top of the dosing through the procedure.