Correct me if I'm wrong, if you have a media fill GB, it become your Bio filter and among other things lowers your pH level.

What is the size of your GB? Ratio between GB and FT has a direct link on your pH levels.

What are your other readings, ammonia, nitrates, nitrites and temp.?

This maybe way off base, but if your FT to GB ratio is off is it possible to circulate a small potion of your FT water into / out of your pond? Like a constant water exchange, bring pH to your system from your pond and using your Bio filter to benefit your pond as well?