So i've been growing a couple of plant's capn style (not important you'll see why) but i've been essentially growing in rockwool mini cubes and have had a hell of a time keeping my ph some become too alkaline daily. I'll water in at 5.7-5.8 and my res will be 6.1-6.2 after one watering. This stumped me for awhile so I went back to basics and ran a best of a test to eliminate variables.

I presoaked a 6inch net pot worth of croutons in 5.5 water as called for by grodan for 2 hours to make sure all the lime from the manufacturing process was out. I did not place any plant in these cubes but gave it a 5.8 watering the next morning and checked the runoff and noticed immediately that it was 6.0. I gave it a couple of hours to dry and ran more 5.8 through and still 6.0 coming out.

Grodan claims without the lime these cubes should be pH neutral but they seem to me to be leeching out the acidity immediately in some strange way. I understand this drift is ok plants like a range of pH to absorb all nutrients blah blah my issue is that I want to feed multiple times a day but can't because recirculating this water continues to rise the pH so res change happens and input water is 5.8 after I run a cycle the res is now 6 and with one more watering its now 6.1-6.2 and with another 6.3. I could add pH down and I have for a lot of this run on an almost daily basis to maintain and i've since switch to just drain to waste to keep her healthy and take the hit on the nutrient costs but seems crazy to me why this is happening with a medium thats supposed to be pH neutral.

Have others experienced this? I'm using RO water, there is no algae in my res or in my test environment without a plan I can simply pour 5.8 water or 5.8 nutrients through these cubes and immediately see some acidity removed.

Any advice is appreciated as I would love to continue use of the medium. I had amazing root growth and it was very clean and easy other than this pH issue.