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I know I have to de-chlorinate the tap water for water changes. What I don't know is, if I'm making a 25% water change, do I add the stuff to make the water safe for the fish before I put it in the tank or afterwards? And how do I know how much to put in the tap water? They come in tablets that look like alka-selser, and one tablet cleans 10 gals. of water. How much do I use for just a partial water change?