Check your nitrates...they are sensitive to nitrate. If nitrates are good, and assuming you don't have readable ammonia/nitrite, I would look at iodine levels. This isn't usually something you have to worry about as long as you are doing regular water changes, but shrimp/crabs do require it for moulting and general health. Don't dose any iodine without checking the levels first though...it's very easy to overdose. Overdosing it could wipe out your tank. I would probably do a water change first and see if the shrimp improves though.
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