Who Gives A Fish Forums

1300l dirt/sand planted tank here, 150l sump.
My tap water's a real challenge - PH 7.8/8, KH11, GH20.
I'm trying to avoid another RO unit, so I've been using collected rainwater to hit my target of PH 6-7, KH 2-4, GH 4-6.

I've been doing water changes for a few days now, and I've managed to bring the tank water down to PH7.4, KH6, GH12.
My question is, is there a delay in the PH dropping, or does it balance out within minutes or hours of water changes? I'm worried the PH might suddenly swing or become unstable when I reach a certain water softness. I've halved my KH and GH, but the PH has only come down 0.5 - I thought it'd drop quicker, especially since the rainwater tests at 6.6.

Any advice would be great, just want to make sure I don't shock the tank.

The pH, GH, and KH are all connected, but it's not as straightforward as it sounds. The GH and KH are still pretty high, at 12 and 6 respectively, so they're buffering the pH and preventing any major fluctuations. However, with all the biological processes going on in the tank, it's hard to predict exactly what will happen.

When you dilute hard water with rainwater, the changes are proportional. For example, if you mix equal parts tap and rainwater, your GH and KH will be cut in half. The pH, on the other hand, will do its own thing based on the new GH and KH levels, as well as other factors like dissolved CO2.

It's crucial to prepare identical water for each water change. If you don't, you'll likely end up with a bigger swing in parameters than you want, which could stress out your fish. I'm sure you're aware of this, but it's always worth mentioning.

I'll keep doing the water changes then.
I've got an auto water change setup from a rain barrel, but the auto top off is still from the mains supply. If I can work out the daily evaporation loss, I can set the auto water change to replace double that amount with rainwater, that should give me a good starting point.