If you want a pH of less than 7 and some KH, 50ppm or about 3 degrees,
it's easy.

Use CO2.

Add enough baking soda to raise the KH to 3 degrees.
Add enough CO2 gas **ONLY** to lower the pH to 6.5 and keep it there
within about .2pH units.
This is what you want to do for both fish and plants.

That's all you need to do.
If you want to understand WHY, well then you can spend a few hours
boning up on the topic, but you do not need to do this to keep
plants/CO2 fish etc.

Just do what I said.
The HOW is easy enough.

Regards,
Tom Barr