Update:
18 jan: seawater (straight from the sea) is in the tank.
19 jan: seawater temp. is 25.5 degrees Celsius, density is 1023, i put in the raw shrimp.
20 jan: seawater temp. is 25.5 degrees Celsius, density is 1023, i measured 0.5 mg/l ammonium.
22 jan: seawater temp. is 25.5 degrees...