My guess would be dissolved organics in the aquarium (decaying plant and/or animal matter, uneaten food, fish waste, etc) are causing the pH drop. If that's the case- larger and more frequent water changes should help remove these from the water and keep your water's chemistry stable.
Out of curiousity- do you know what your
KH and
GH levels are? Higher levels mean more buffering ability- which leads to more stable pH values. If your hardness levels are low, there won't be much buffering capacity and your pH can drop or spike suddenly- with terrible results.