Hi there,
I'm not familiar with Stratum, but I believe that it is a buffering soil substrate, right? It will therefore continually reduce the KH to near-zero until such time that it's capacity is reached (at a guess somewhere in the 3-6 month range with your tap water, if it's anything like ADA Amazonia). If you're adding KH at water changes then you are fighting against the soil, hence the pH swings. Active soil substrates are generally best used with 0KH RO water; the soil takes care of the buffering so you don't need KH and it can be zero (or thereabouts). But the resulting pH will be on the acidic side (pH 5.5 - 6.5 range depending upon the substrate). Adding crushed coral is similarly fighting against the soil and will quickly deplete it's buffering capacity.
If you're looking to deplete the soils buffering capacity and get a more neutral pH in the tank, then it will take a little time. Regular water changes with your 4 KH tap water will do the job, but will probably take a few months. Rather than keep doing water changes, you could just add a buffering (KH+) product to the tank every day or so. Adding little and often will help reduce KH / pH swings to some extent. I wouldn't use crushed coral personally as not so easy to control and predict. A KH+ product like Seachem "Alkaline Buffer" is much more controllable (non-phosphate buffers usually recommended for planted tanks). Either way, it will take a while (month or two minimum) for things to settle down.
The soil is doing what it is supposed to do - reducing KH to zero and buffering the pH down to a slightly acidic level. You need to 'break it' with constant KH until the point that it's buffering capacity is exhausted if you want the KH to stay above zero and the pH to balance around neutral.
Hope this helps....
James
