Are these stem plants? If so, are you putting them in still bound together with the metal weight? If you do that they will start to die off from the bottom and work its way up. I have the worst luck with stem plants because even without a weight they do that.
I highly doubt its the shrimp. Sound more like lack of some nutrient or low light problem.
I agree with dizz. When I get stems I remove them from the metal weight and cut off any dead or unhealthy bottoms and then I plant each stem individually. If the stem has a healthy root system you should cut the roots back to about 1 inch, it will promote new growth.