I think I've found the problem. It's the copper from the pellet I've tried to feed my micro-fish. The fish don't eat those and the food decay in the bottom, maybe that's where the copper leak and killed my shrimp.
I just did a water and clean the sand. But I couldn't do a 100% cleaning as that will take more than 50% water change and harm the shrimps.
SO I did like close to 50% water change and add a layer of new sand on the old one ( I sucked a lot of old sand out).
After water change one of my shrimp is acting weird, I'm afraid he will die soon :-s
He's like in some kind of concussion. He was hanging near the tank surface and when I touch him, he falls like he's dead and lie still on the plant. Seconds later, he turns around and swim somewhere else
The symptoms the shrimp is showing are classic for shock. Did you match the pH, gH, kH and temperature of the new water exactly to the water in the tank? How much water did you change?
The small amount of copper in food is unlikely to be the cause of the deaths I'm afraid. Even shrimps need a small amount of dietary copper and Hikari shrimp Cuisine (a special shrimp pellet) contains copper as a trace element.