well.. what i did to help raise o2 in the tank so far is, point powerhead returns toward the surface for more suface aggitation and add a small powerhead for dead spots in the rear of the tank. i can try to open a window but that might be reaching.
im not sure what else could cause the ph to drop like it is. like i said, besides magnisuim being low, which can cause loss of calcium and k/h, and i havent seen signs of that yet. but with my kh being around 9 and calcium at 400 u would think the ph wouldnt drop, unless there is something decaying in the water causing waste.