If you don't top up with RO water or distilled, only do water changes/top offs with tap water, then that could cause your GH and KH to rise. Water evaporates, minerals don't.
If you want to confirm if it's the soil or not, you could place some in a bucket with fresh tap water and let it sit for a day to a week and see if the parameters of the water change.
The shrimp might be getting too much protein. Many "algae wafers" aren't algae based at all, but rather, algae infused. You might need to double check the ingredients and see if you can find a true algae or vegetable based food to feed at least twice a week, then your protein food once a week. Failed molts could also occur if there's too much protein in the diet.