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It's good you tested the rocks. There might be something else causing KH to rise. What substrate do you use? What filter and media do you have? Sponge air filter or External filter? I have seen cheap glass noodle filter media causing KH to rise when tested in RO water. Reducing your fertiliser dosage also helps. If there is nothing else in the tank that is causing KH to rise, then it must be the ferts. Using straight RO water for your water changes to reduce KH is a right path to take.

I believe the rocks are banded jasper. Previously when I first set up the tank, I was worried about the rocks. I have done previous testing with them before. I don't have hydrochloric acid but I've done the vinegar and water soak test. I had a test rock in RODI for a month with no change in GH and KH. There were no reactions to vinegar. The rock was marketed for aquatic use but it doesn't always mean it's correct. Sometimes I do question it though. I previously moved some rocks higher out of the substrate to help fix the scape and there was a dividing line in the color of the rock. The portion originally above the substrate had darkened and the part that was previously buried in the substrate stayed it's original color. The only other thing that comes to mind is that maybe I have been overmixing my re-mineralized water. I could have sworn that when I originally made the mix to 180 TDS in 1 gallon of water, it was 6 GH and 4 KH. I did some mix testing yesterday and at 160 TDS, it was 6 GH and 3/4 KH. I did a 10% water change and replaced it with straight RODI water. It brought the KH down to 5/6 but GH also down to 5. I will do another 10% water change next week and probably add in mostly RODI and a little re-mineralized to keep calcium and magnesium content in the water for the shrimp. I think the KH rise could have been part of the issue. If I recall, I saw a shrimp laying around and twitching, like I've seen in a failed molt. The higher KH had totally skipped my mind and I went straight to thinking it was a food issue due to the ton of shrimp in the tank and light supplemental feeding. Though maybe it can be a contribution to possibly shortening their lifespan.

I use Fluval Flourite, which should be inert but should have some cation exchange capability. I use a sponge filter but it's a design that has a small filter media chamber. I filled it with Fluval biomax which I use in my other tank with a canister filter. I found another dying shrimp this morning. Seemed like a young adult male, might have been a failed molt but I can't tell for sure. I take it that KH reduction won't necessary see immediate effect since the shrimp have already developed their current shell? This would be something that takes time to fix for the shrimp? Aside from the fertilizer, the only other thing I can think of is a contaminant got into the tank. I would think that affects all the shrimp and not seemingly select individuals.