You still need PO4, not much, but some.
Some GH booster is also par for the course, see the article and stick to the advice contained in the article.

You'll dose once a week at most and then pretty lightly.
Add algae herbivores, SAE's would be good, Amano shrimp etc.
I'd trim off most of the infected leaves, add fresh plants, cheap ones etc till the other's slowly grow back etc.

Regards,
Tom Barr