Yep, Iron deficiency can cause darkening. Seachem Iron is a good option if you're not already using it.
why are my roots turning black?
Re: why are my roots turning black?
Another thing to consider is temperature. Anubias prefer temps between 72-82°F. If your tank's too cold or too hot, it might stress the plant.
Re: why are my roots turning black?
I've had success with Anubias in lower light tanks. Maybe reduce your photoperiod to 6 hours and see if that helps. Too much light can encourage algae on the roots, which might look black.
Re: why are my roots turning black?
For real, though, if the roots are mushy, cut 'em off. Ain't no saving that. Healthy roots are firm and white-ish. Don't let the dead stuff mess with the rest of the plant.
Re: why are my roots turning black?
To add to the substrate discussion, anaerobic bacteria can produce hydrogen sulfide, which is toxic to roots. If you smell a rotten egg odor when stirring the substrate, that's a clear sign. Increase water flow or reduce substrate depth to mitigate this.
Re: why are my roots turning black?
It's great to see so many helpful suggestions! Remember, plants can take time to recover, so don't get discouraged if you don't see immediate changes. Keep us updated on your progress!