Coco is not soil, the pH of the medium isn't as important as the pH of the nutrient solution you are feeding it. If you are having problems keeping the nutrient solution pH from drifting:
pH stability can have a lot to do with carbonate hardness. If there is no carbonates in the water (soft water) the pH will drift up and down, but ultimately fall low. If the carbonate hardness is too high (hard water) the pH will be very high and be difficult to bring down. If you have a good hardness, on the softer side (2-4 dKH) the pH will remain stable even in a slight acid (5.5-6 pH).
But again, don't worry about the pH of the coco or runoff, as nutrients are consumed and ions exchanged by the coco the pH of the coco and runoff will change. This changed pH means nothing to you as a grower. It's only the pH of the nutrient. Would you be as concerned about the pH of a pot of hydroton?
As to you asking if there's a product that will help keep pH stable, yes. You can buy calcium carbonate and use that in your reservoir if your res is having pH drift due to water being too soft. If you run beneficial bacteria you can put some crushed coral in a mesh bag and sink that in your res, it will slowly increase the dKH of your water and also play host to colonies of beneficial bacteria.
Unlikey a tap water user needs to add calcium carbonate unless their water is super soft. Most arent. If you live in seattle... Well you know your water is soft.
Check your cal hardness before adding more. Hot tub test stips do this.
i use tap water
Ok and yeah you can look up water reports or you can be exact, like say cross referancing a tds meter and test strips. You were just preaching to me about exact science, or your understanding of it anyway.
Why are you here, arguing yet again on a thread discussing things you dont believe in, like checking run off in coco? Its an insult to all the coco masters here...
Tap water works very well all by itself. I use my 8.3 ph, 350 ppm tap + GH Flora and when i mix it comes out exactly @6.