You can dose when you do the water changes directly to the plant filter.

There's no flow for a few minutes, the longer, the better.

I'd add about 5-10ppm of KNO3 once a week.
1 ppm of PO4
10mls of Traces.

Etc...........

You'll note you are after a balance of low nutrients, but not so low the plants suffer. So that is a balance.


Regards,
Tom Barr