What Zoidburg is trying to get at with your ADA is that you are using a substrate that is specifically formulated to buffer your pH into a specific range (mid six ish). Meanwhile, you are putting tap water with a mid-high kH into your tank (carbonates will buffer up, closer to high sevens, maybe eight). Every time you put in tap water, your pH and kH swing around until your substrate wins (for now) and your pH and kH drop. Eventually, it will lose the ability to buffer and your tank will migrate towards your tap parameters. Big changes in parameters are bad for any colony, which is why Zoidburg is suggesting you either go full inert or full RO and stick with either card or neo parameters.