My guess would be that the one-time shock would probably take care of it. Then, with cya of 20, just maintain cl readings between 2-5. As far as adding a little more cya or not -- that is up to you. It certainly won't hurt to bump it up just a little. Or you can just wait and see how much trouble you have maintaining a cl reading in there with cya of 20. You may be fine. And, if you do have trouble, you can decide a little later to add some more. Up to you. Glad to be able to help.