I have a problem with trying to reduce the pH in my ponds. However the "water" counteracts any acidifying processes. The water I use to top up the ponds is from a bore (ground water). The water itself comes out at a pH of 7.4 and with alkalinity of 312 mg/l. Once it sits in the pond the pH goes up to about 8. It was going higher but I've been adding a lot of dead leaves. The fish and nitrifying bacteria don't seem to be helping much with acidification. I think their input into that cycle is probably minimal. Soil in the pond or concrete walls (if they have them) are more significant factors.

This is still just a roughly 15x3-foot plastic cylinder with an open top anda little sand and gravel on the bottom for bacteria to live on. But I've been reading some more and have some thoughts.

Apparently losing CO2 and gaining oxygen increases pH, slightly. This occurs at the surface during typical gas exchange. Also, plants pull CO2 and dump oxygen during the day as part of photosynthesis, increasing alkalinity. At night, when they reverse course and respire, they bring pH back down, the acidity/alkalinity cycle daily.

I was thinking, with a constant rise, fall, rise, fall cycle, any peaks or troughs in pH will be exaggerated, temporarily but frequently; this should have an effect on concentration differences for gas exchange at the surface, and apply more pressure to move back towards neutrality the further off it is, in either direction. I suspect that as long as I keep sufficient live plants in there, but not so many as to clog up the surface and inhibit gas exchange, extremes of pH cannot occur without a drastic change elsewhere in the system.

This idea may be wrong, but it fits with what's seen in real ponds, too. Many many plants, both above and below the surface, they take care of their own balance, and it only seems to be the really clogged ones that get nasty.