That stuff is a real problem for me at times, especially on my alternanthera reineckii. I really hate the thought of getting another test kit but I suspect my phosphate content may be on the high side. If it gets any worse, I may do a blackout or selectively blackout the long stems on my alternanthera with a small black plastic bag. Maybe that will slow it down until I can get my water balanced. My tap water is fine to drink but it leaves a lot as far as growing plants is concerned. RO water is a major pain in the backside and has its own problems as well.
pH: 6.6 to 7.0
Ammonia = nil
Nitrate = 10....perhaps
2 W/gal for 10-12 hours/day
Aquarium Pharamcutical root fert.