Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
I wonder if the tank is so tall that the lower parts are not quite bright enough to keep the plant leaves going, so the algae is growing there.
The 'string' might be roots. Some plants will grow a lot of roots that way, some grow few or none. I take advantage of that when I trim, burying these roots to get the new plant anchored a bit better.