What kind of substrate are you using? Is it a FOSW or reef? There is substrate out there That will stabilize the PH to. Another way would be a chemical made by Seachem that keeps the PH around 8.3 . You can get it at petsmart. Before you go adding more PH chemicals do a PH test through out the day. What I mean by that is your PH will be higher in the morning and evening then during the middle of the day. Alkalinity can cause your PH to drop These are examples that will contribute to a drop (1) excess carbon dioxide (CO2) from respiration caused by lack of sufficient gas exchange, (2) nitric acid from biological filtration (nitrification), and (3) organic acids from metabolic wastes. I have live sand as substrate and my PH stays between 8.2-8.4, the only time I have have to add PH is when adding water. Do not add any more fish till you fix the PH problam. Hope this can help you out.