Help, I have high ammonia levels and low PH
I have had my 55 gallon aquarium for a little over a year now. I have 3 zebra danios, 2 algae eaters, a loach, 2 striped catfish, 1 blue neon tetra, and 2 goldfish. I had more neon tetras, but the goldfish ate them. I know that normally goldfish do not belong in my tank, but they have done fine for six months now. I have an Aqua Tech filter. I keep the temp around 80 F. Up until recently, all of my chemical tests had been normal. But recently, the ammonia test starting showing positive and the PH is at about 6. Previously, my PH had usually been high (around 8), but the fish seem to be doing fine. I haven't done anything different to cause this change. The fish store where I buy my fish at recommended that I clean my tank once a month and add some freshwater aquarium salt, some ich treatment and Nova aqua conditioner and fish protecter at each cleaning. I have been doing this for probably about six months now and have had no problems before, but now I have a high level of ammonia and my PH has dropped from its normal high of 8 to a low 6. I added some zeolite granules to reduce the ammonia and put some proper PH in the water, but I am still not showing any change. The PH went up for a couple of days and went back down. The ammonia level lowered, but still has not disappeared completely. Does anyone have any suggestions for me before I lose my fish? Any advice would be greatly appreciated.