Ph/kh problems in my 15g planted tank

On Friday I finally dove in and started up my 15g. I did the hardscape,planting and hooked up all the equipment which includes a pressurized co2 system with a ph controller.
the problem starts with the fluval stratum substrate this stuff really lowers the ph and kh of the tap water , my water is about 8.5 ph and 4 kh out of the tap and after filling the tank and many hours spent planting my ph was close to 7.0 or slightly less!

I busted out the test kits only to find my kh drops have runout, I assume that some natural acids in the substrate have used up the kh in the tap water resulting in a lower ph

What would be the best way to achieve a 30ppm co2 ? should I set my ph on the controller really low or should I do a water change to see if the ph will return more close to my tap water or should I add some sort of buffer to raise the ph/kh so there is room for co2 ? Any help is appreciated and I give more information needed to figure this out !

Do you have fish or other species in there already? That would determine my course of action. Don't forget that the CO2 is also likely to lower your pH.

You should really try to tune it with a co2 drop checker. Those will be more helpful than measuring ph. However if you can rig your pH probe into a drop checker, it will give you perfect control over co2.

The tank only has 2 zebra daino the filter is mature from a larger thank that has been running for years!

I am aware that the co2 will reduce the ph and this is the problem the substrate had already lowered the ph to where it would be if the co2 was at 30ppm the plan was to let the co2 run until the ph was about 6.8 then my ph controller would maintain that ph and 30ppm co2 but now my tank water is softer than my tap water if I were to try to get 30ppm co2 the ph would crash and become unsuitable for the planned inhabitants

Should I add a kh buffer or just keep doing water changes to get the kh back ?

I am not sure the water changes will help as fluval stratum is known to soften the water and it is likely to be active for some time.

I am not in favour of chemical additions. What about using calcic rock in your scape or put a bag with some shells in your filter? This might do the trick

pH is really a very inaccurate measure for the tank as pH fluctuates a lot. I have heard the fluval substrates tend to modify and keep kH low. It is most commonly used to help keep Cardinia species of shrimps. I am also sure there are many case of where the fluval substrate is used in conjunction with CO2. I would worry more about controlling the CO2 ppm more than controlling the pH parameter. Again I believe measuring a drop checker and controlling your CO2 based on that would give you much more stability.

So today I got some kh drops and re tested my water in my tank and my tap water , I'm not sure if they change the water here seasonally but it has gotten harder since I tested it back in the summer it now reads 8.5 kh and ph is off the charts above 8.5

In my tank with the co2 and the fluval stratum I have 5 kh and my ph is 6.6 down from about 7.0 I think I'm gonna back off the co2 and run the water as is without adding buffer how manny bubbles pm should I use for the 15gallon ? Also how could I make some 4 kh water for the drop checker ?

You can get some RO or distilled water to dilute your tap water or, purchase 4 kh water from a reputable source.

Thanks for the help guys