You need to find a way to introduce some oxygen into your system to get your ph to rise. I'm not one to ever check my ph but I have a 150 gallon topless system with 2 sumps and 2 skimmers, 4 koralia 4's. There's plenty of oxygen going on here so my ph stays up. Try aiming a powerhead at the surface of the water?? Do you have a skimmer on there. It may be worth getting a small skimmer just for oxygen purposes.