Sorry to hear you keep losing those shrimp. I know that the caridinas are very sensitive to water conditions. I think Zoidburg gave some good advice about the tannins. Tannic acid will help lower your pH temporarily. The place I started to purchase Black King Kongs recommend keeping pH between 5-6. Not so much for their preference but to deter diseases.
I imagine it's pretty hard to distinguish disease from parasite on these dwarf shrimp. I many cases, it's too late to treat by the time you notice. I've seen them scurry about sideways and their mouth parts move at an excessive rate before they died. In my case, it was some form of necrosis. I pegged it to the LFS water quality. I would be ready to cull a few more if they start acting funny, before the entire tank becomes infected.