I would start by asking why pH is high to begin with...
pH is indirectly tied to alkalinity. Low alkalinity pushes the tank towards low pH. High alkalinity tends towards a higher pH. What's your current alkalinity?
Your next biggest factor is additives that affect pH. Kalkwasser pushes it up, 2 parts addtives using anhydrous baking soda as a component will push it up. If you're using one of these, reduce the amount (your alk may be high). If you're doing a DIY 2 part with baked baking soda, consider using straight (non-baked) baking soda. It has a short term effect of lowering pH.