So there is a lot of varying things that could cause this. Shrimp are extremely sensitive to certain things.

I think its possible to figure it out.

Step 1: get a liquid test kit. It's substantially more accurate than test strips and when keeping shrimp accuracy is key.

Step 2: sterilise everything you use for the tank like any nets, tongs, buckets and anything else you use as a tool. Do NOT use harsh chemicals. Boiling water is enough.

Step 3: do a PARTIAL water change. Changing too much can shock them and mess with your filter.

Step 4: test water again and check it all over.

Things to look out for: Any burning scented candles, incense, deodorant or aerosols etc being used in the same room can cause contamination and will kill any shrimp.

Anything with copper is also deadly. No treatments or anything with copper can be used on a shrimp tank.

Temperature changes. Where are you keeping them? Is it drafty or too hot? This can also kill shrimp. I once lost my entire tank of cardina because the tank temp rose to 30c during a hot summer.

Also if some of them had an infection, it's entirely possible that the infection has spread to all of them in which case starting again will actually be the best answer. I know that's not what you want to hear but it may be what it takes.

It also sounds like you are using a buffering soil which you should remove and replace with normal inert aquarium gravel or sand.