I am just kidding, well sort of, about the pot growing.
I am a hydroponic/aquaponics type; the humor is that it is the pot growers that are on the leading edge.
One of the real differences is the level of fertilizers, which are much higher in hydroponic systems, what I am trying to say is it is not Nitrogen lock out.
in the Nitrogen cycle in your tank if you are reading any
Nitrites or ammonia at all with hobbyist test kits.
Best guess is that you need to increase dosing in general, in particular phosphates.
Fish food is not enough N, P or traces in a system with more than modest planting. Even in planted tanks with significant fish, populations’ fish food should not be counted on for more than 20% or so of plant requirements. Salts provide nutrients at far lower risk to the system.
As far as the Black algae, which is actually Red Algae, Rhodophyta, Red algae like Anubias with the thick slow growing leaves.
- Use a toothbrush and brush away as much as you can,
- unfortunately some leaves may not be saved,
- brushing and direct application of 3% Hydrogen peroxide OR 20-ppm Potassium permanganate,
- actually if the plants can be removed, a 15-minute bath in 20-ppm PP
- after srubbing and scraping as much algae as you can.
Make sure the Anubias plants have good water circulation and some shade.
A major water change is a good time to do this. Before returning the water wipe down, scrape and scrub all hard surfaces, then with a Fleet Enema moistened paper towel or soft cloth wipe the glass and hard surfaces.