Hard brown stuff killing my plant

[Magma4life]

So I've had my plants in a small glass fishbowl while I medicated my tank cause I read meds aren't good for plants. I change the water in the bowl every day but lately I've noticed some hard brown stuff right where the stem and leaf meet on my anubius congensis. It's really hard and won't come off when I scrape at it with my fingernail. There's also a small blackish spot on top of the brown stuff. The roots are turning brown as well. I've also noticed that the leaves where the scum is are turning yellow and dying.Any ideas? The other plants in the bowl, an indeterminate amount of marimo moss balls, are doing fine. In the photos, the brown stuff is on the far left and right leaves.

 

[SaraB]

here's probably not enough nutrients in the water for the plants. Try not changing the water every day. Also add fish food for them, it sounds like a weird thing to do but the plants feed off of the nitrites/nitrates in the water. (or something like that)

 

[Magma4life]

Thanks for the advice! I just put some fish food in there and we'll see if that helps.