Nitrates themselves don't affect pH.
However, the ammonia that is converted into nitrite to nitrate can cause it.
I'd be willing to bet your KH is pretty low (surprising for a pH of 8.2). Sand/substrate cannot maintain a pH unless the water isn't changed. If your KH is low, add some baking soda to get the KH up - it will do 2 things: Keep the pH at 8.2 (if you add enough) and buffer it so it won't drop when the ammonia/etc gets in there.
What's the pH out of your tap?
Say it's 7.
Let's say your tank is at 8.4 - and you replace 50% of your water with tap water. You'd be at pH 7.7.
However, the ammonia that is converted into nitrite to nitrate can cause it.
I'd be willing to bet your KH is pretty low (surprising for a pH of 8.2). Sand/substrate cannot maintain a pH unless the water isn't changed. If your KH is low, add some baking soda to get the KH up - it will do 2 things: Keep the pH at 8.2 (if you add enough) and buffer it so it won't drop when the ammonia/etc gets in there.
What's the pH out of your tap?
Say it's 7.
Let's say your tank is at 8.4 - and you replace 50% of your water with tap water. You'd be at pH 7.7.