PH keeps dropping.

[Waterboy]

I have read several threads on PH and have a question that still stumps me. My PH in my system drops from a 7.8 to a 6.0 in about 5 days. I have a 500 gallon system that cirrculated for 1 week without fish and before cycling. It did not change then, so I am confident its not the grow media or water.
I have about 50 young fish, and about 50 minnows and a few crawfish in there right now. A have a piece of coral from a salt water fishtank that I added after the PH started dropping, and the rest is plastic fake plants. I struggle feeling them pellets due to the catfish only eating at night, and crappie not eating good, so there is leftover food in there, but the crawfish take care of that in time.
I use calcuim carbonite, and potasium carbonite to raise it when needed.
I have had the system going for a few months now. I guess my question is could this PH dropping still be from bacteria multiplying, or something else?? NH2 and nitrites are always 0. Nitrates stay around 40PPM. I do add new minnows that I catch from the ditch?? Anyone have any ideas?