I don't think the OP is adding any alkaline buffer (NaHCO3), so the KH reading is probably all coming from the tap. Much of the pH alkalinity is probably also coming from the tap. Tap water can have many types of alkalinity components and our test kits measure total alkalinity, not just carbonate. For example; if the pH is above neutral (7.0), phosphate could be a factor in the pH reading and tap water often has high phosphate levels. Add the CaCO3 from the different possible sources (Eco-Complete and, maybe, residue from previous tank and rocks) and it might be enough to push pH higher than desired. Once the tank is stabilized, it will probably drop somewhat.
BTW: for your bucket tests of the rocks, also put some of the Eco-Complete in a bowl to satisfy yourself about that, as well. The bucket tests will also tell you about GH levels that might be coming from the rocks.
I'm not sure about CO2 in tap water. It depends on your water company. Some do add CO2 to push pH down, but that would gas out in hours after leaving the tap (the pressurized tap water can hold more injected CO2). The places where I have lived do not do so and, therefore, has no more than ambient CO2. you would have to check with your water company.
pH can be moved dramatically by little things in RO/DI water (plant activity or small CO2 changes), which is why it needs to be buffered. I suggest bringing the dKH up to about 4 and take a reading in 24 hours to see if there is a change.
Regardless, is there a reason for the focus upon pH? pH of 7-7.4 is not unusual. I wouldn't worry about it, unless it moves dramatically without CO2 changes. Longer term, I'd be more concerned about achieving a stable pH with at least 4dKH, other than any variation caused by CO2 injection. Is/will CO2 be injected?
You also mentioned that the "softening the water part is done" based upon KH, but KH has nothing to do with soft water. If you are trying to adjust your water softness, you need to look at GH, which is a function of Ca and Mg and also will drive a lot of your TDS measurement. Right now, you are relying upon your tap to mineralize your RO/DI, but you could easily do this with GH additives if you want to adjust the softness/hardness.
Last edited by Deanna; 10-11-2017 at 01:35 PM.