The short answer
A shrimp sitting still is usually doing one of three things: moulting, reacting to temperature (too cold and it goes sluggish), or it’s unwell or stressed by water quality. Most of the time a motionless shrimp is completely fine — moulting and resting are normal. The trick is reading the signs: look for tiny antenna or leg movements, check for a shed exoskeleton nearby, and test your water before assuming the worst.
Moulting — the most common reason
Shrimp grow by shedding their shell, and around a moult they often go still and hide. You may find a clear, empty shrimp-shaped shell lying in the tank — that’s a moult, not a dead shrimp, and it’s a sign of a healthy, growing shrimp. Leave the shed shell in place; shrimp eat it to recover minerals. A shrimp will usually be back to grazing within a day.
Temperature and stress
Shrimp are cold-blooded, so if the water is too cool they become slow and inactive. Check your thermometer — most cherry shrimp do best around 20–24°C. Sudden swings, or being added too fast, also cause a shrimp to freeze up. Always drip-acclimate new shrimp slowly to avoid the shock — see why did my shrimp die after a water change.
Water quality and illness
If a shrimp stays motionless for a long time, isn’t moulting, and others look off too, test the water. Ammonia, nitrite or copper are the usual culprits — confirm with a test kit. Keep parameters stable and copper-free; the full routine is in how do I keep shrimp alive.