Lynda is right. BTW, baking soda (sodium bicarbonate) will stabilize ph at 8.4. If ph is above 8.4, addition of baking soda will lower it, but not below 8.4. Conversely, if ph is lowerer than 8.4, baking soda will raise it but not above 8.4. Because the natural action of a biofilter ordinarily lowers ph, it is a good idea to regularly add a little baking soda. You want to avoid a ph "crash" at all costs.