Chances are, your substrate is pulling GH and KH from the water column, which is why your GH and KH are lower than your tap water.
If this is the case, then I highly suggest switching to remineralized RO water using GH+ only remineralizers.
I've already explained that using tap water with a buffering substrate will cause instability in water parameters. Now you can see that for yourself. You add tap water, the pH, GH, KH and TDS go up. Your soil then works hard to remove KH out of the tap water (and potentially some TDS), which lowers (at minimum) the pH and KH, potentially even the TDS. Each time you do a water change, there's going to be a fluctuation in water parameters - until the soil can no longer buffer the water column. At which point, your pH will climb above 7 and remain there. (using RO water + GH only minerals means that the soil can last longer and you end up with more stable parameters)
The more tap water you use, the faster your soil will deplete itself and thus the pH will rise. That crushed coral? Also not good for the tank due to the buffering substrate.
Definitely not a tank good for shrimp.
Unless this is a different tank or you only plan on keeping plants in it? In which case, disregard that info.