This quantity every 1-2 week
Some of my shrimps were starting to die after having them for a few weeks. And found out that one of my friend had Ro water.
Test RO water, Improperly maintained RO systems can throw off Bacteria as well.
Second, were these imported or domestically raised shrimp?
Do you have an estimate for how many shrimp in the tank, you said they breed and you have bought 80.
Were they from the same seller?
What do you dose?
How often and how much are the water changes?
To much changes in water parameters can cause stress to shrimp as well. Its better to go slow with changes unless something catastrophic happens.
Do you treat for anything?
Was the tank new when you set it up or was it used?
Copper leach from prior treatments can cause issues if parameters change in the tank.
I know alot more experienced shrimper's can chime in these are just a few of the issues I know about.
Are there any other inhabitants in the tank (fish, etc).