Possible reasons for shrimp deaths..
1)The new plants were treated with anti-snail stuff and you did not wash the plants before planting into your tank?
2)Did you stir up the base fert while uprooting the old plants in the tank? If you did, possible spike in the level of ammonia from the uprooting task caused the shrimps to die off.
3)Removal of settled plants in your tank cause the amount of available O2 to drop inadequate levels...as plants, fishes, shrimps and nitrifying bacteria compete for O2 at night, the level probably drop to risky levels thus causing all your shrimps to die off ..(shrimps are very sensitive to low O2 and high CO2).
Okay, next time when you uproot any old plants from your tank, it is always best if you can do a large water change as it removes the junk from the tank and adds lots of O2 back into your tank with fresh water.
Plant Physiology by Taiz and Zeiger
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