White Ring Of Death. Unsuccessful molts of a shrimp
"Bad, or failed molts are usually linked to too large of water changes, a poor diet, or wrong parameters (GH, KH, PH). When shrimp are lacking the key elements of their parameters, they are unable to grow, and shed healthy exoskeletons. You may notice this in the early stage as the “white ring of death” which looks like a solid white band around the shrimp where the head meets the body. A healthy shrimp will split just at the top of it’s head, allowing it a clean break, or molt, out of its exoskeleton. When the ring appears, it makes doing this more difficult, and a shrimp may die in the process of trying to molt, because it can get stuck while trying to do so." - Buceplant
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The bloody mary cherry shrimp I kept were prone to this. This was the first time I had kept neocaridina shrimp. I couldn't figure out the cause and went from 15 shrimp to now 2 remaining. I tried Mineral blocks, organic salad leaf diet, reducing water changes to 10-15L a week, and even introducing a cuttlefish bone to boost the hardness all of which didn't work. I've since bought some lower-grade cherry shrimp and had no deaths in the month and 2 days I've had them now. I still struggle to understand neocaridina shrimp. I've kept amano shrimp much longer and never had any issues like what my RCS had. My guess is that water parameters caused the problems as I have softer water.