Go buy some fish fertilizer. Many hardware stores like Ace and True Value and Walmart sell Alaska Fish fertilizer for plants. It will lower the ph. You could just mix it at 1/5 strength and use it every watering. I have 6.8 ph spring water and when I mix it with 1/2 strength it drops the ph down to 5 and I have to ph up with Baking Soda to ph 6.7/6.8
Go to a pet store or maybe even Walmart again and get some ph down for fish aquariums.
Lemon juice
From MyNameIsStich infirmary notes..Note: Blood Meal, Dried Blood, Guanos, Kelp Meal, Cotton Seed Meal, Peat Moss, Sulfur and fish meal are all acidic and can bring your ph down, so if you add these please monitor your ph when using those.