Sorry late chiming in here. Have you tested your tap water?

Most regions in the lower mainland have a sub 7 pH (mine is 6.5 ish) out of the tap. Since bioload in most FW tanks tends to deplete alkalinity and lower pH, Im thinking that you might have something in your tank that is releasing carbonates...

What sort of substrate do you have? Some types of mixes have bits of crushed coral or shell to 'stabilize' pH and coincidentally, pH around mid 7 is where they often balance out to IME.

I have kept a school of cardinals for over two years at pH around 7.8 and they are thriving.

You only need to lower the pH to try to breed them. When I did that, one of them died.