Just wanted to add a note to Watermom's post....if you add the CYA through the skimmer, and then backwash the filter before the CYA dissolves, then you are washing the CYA out and will need to start over. IF you anticipate frequent backwashing, then I would put the CYA in an old sock and hang it in front of a return so that backwashing the filter won't waste the CYA.

Janet