Update: As of now he’s still alive and active even after being put back in the 20 gallon. In my experience if something is wrong shrimp will die quite quickly so idk if his sudden lethargy was just me being a helicopter parent.
It’s mostly a myth that fertilizers are bad for invertebrates, the common one that is brought up is copper but that’s at levels above 0.03 mg/l which would be a significant dose, a 75 litre, 20 gallons, aquarium would need 2.25 mg of copper. That said copper is an essential nutrient for shrimp so definitely shouldn’t be avoided either. The fertilizer you used has no copper listed as an ingredient anyway.
Whatever is going on with your shrimp is likely nothing to do with your fertilizer unless you overdosed by a significant amount.
selgarx 
I dont use any products in my shrimp tanks. They like nutrient lean environments rather than nutrient rich.
Impossible to say which chemical is responsible for deaths, but to be safe I dont use anything
Update: As of now he’s still alive and active even after being put back in the 20 gallon. In my experience if something is wrong shrimp will die quite quickly so idk if his sudden lethargy was just me being a helicopter parent.
It’s mostly a myth that fertilizers are bad for invertebrates, the common one that is brought up is copper but that’s at levels above 0.03 mg/l which would be a significant dose, a 75 litre, 20 gallons, aquarium would need 2.25 mg of copper. That said copper is an essential nutrient for shrimp so definitely shouldn’t be avoided either. The fertilizer you used has no copper listed as an ingredient anyway.
Whatever is going on with your shrimp is likely nothing to do with your fertilizer unless you overdosed by a significant amount.