After using JBL PhosEx ultra, my tank water is coloured brownish-red.
One can assume that JBL PhosEx ultra was not washed out prior to use.
Rinse the net bag with JBL PhosEx ultra under flowing water before using for the first time in order to remove dust particles which cannot be entirely avoided due to transport, etc.
Do a larger water change of about 50 - 60 %, take JBL PhosEx ultra out of the filter and rinse the granules thoroughly once more. If possible, reduce the filter flow rate for several hours, making sure that the oxygen supply is ensured, so that any particles which may still be present can be deposited and then removed with a gravel cleaner.