I am having a similar problem.
I have a 20 Gallon glass (tall) tank. I have gravel, 1 plastic plant, 1 ruin ornament, 2 black moores, and 2 calico fantails. I have an aquaclear 150 filter (b.o.t.), and a bubble wall for oxygen.
I have to change 20% water every week to bring my
PH back to safe levels. It comes out of the tap at 7.5 here, and in a week, it drops to almost 5. My Nitrite, and Nitrate levels are almost unreadably low (very safe zone), ammonia doesn't show in tests either.
Does anyone have any idea why
PH would drop so drastically in a week. Local pet stores can't give me an answer, only that I seem to be the only person on the planet that has his
PH drop.
Hopefully I provided enough info. from what I can tell, I have lots of room for more fish, so overcrowding isn't a factor. From what I have seen lately, could I have too much oxygen? I added the bubble wall, after reading that more Oxygen would increase
PH a bit, but now I am reading here that more oxygen will lower
PH. Which is it, and would there be a more stable solution, instead of adding chemical every week? This problem has only surfaced in the past month. My aquarium has been very stable prior to this, for almost a year.