I have a pH probe in the water that tests the pH. It is hooked up to a controller that shuts off at a specific low and high pH that you can set. As you add CO2 to your tank your pH naturally goes down. If it goes below 6.0 my controller shuts off which shuts off the solenoid to the CO2.

For gravel in my old 29g tank I was using a thin bottom layer of cat litter (pure clay, no scents..Petco brand) and dirt (soil not potting soil or manure) 1" or so, then held it down with sand, and the top layer was just gravel. In my 75g tank it is 100% Flourite from Seachem. This substrate is clay based and is high in iron.

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--->Pictures of my 75g Amazon tank<----