That "black" stuff is cyanobacteria also known as blue green algae. I battle with it as well especially on slow growing plants like anubias. Sometimes i rub it off during a water change. Limitting nutrients also works, mostly feeding less. And yes, like others said you need to expose the rhizome, dont bury it
Once the Rhizome is out of the substrate, give a month to see if it improves any. The black on your leaves, as mentioned, is algae. Rubbing it off seems to be the only way to get rid of it. As a preventive measure, small algae eaters help such as Oto Cats or BN Pleco.
I always had trouble getting my plants to stay put and i ended up burying the anubis plants up to the rhizome. Eventually the roots pushed it out of the ground.
Based on my experience yesterday at home I can recommend an answer to thread title.
Yes! Have a cover if you have kids , my lil man dumped all his toys and some kitchen utensils in the tank to make it more " colourful "
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Yea for the Anubias algae... The lighting plays a part too.... And also cut off few leaves at the point where the leaf starts not the stem thing just the leaf... I did this last week... And boy my Anubias is shooting new growths left n right.... And ferts is all u need... Co2 supplement can work wonders too... Cheers