Agree emphatically! I would add this: I have a really big swordplant that constantly produces leaves and runners. When the oldest, biggest leaves start to show their age, they are removed. They always have holes, yellowing dying tissue, and develop skins of various green algae, as well as BBA along their edges. The plant wants to be rid of senescent leaves. When trimmed up, and the plant is kept tidy, there is no algae to be seen. This is a clearly drawn example of the general idea, pointed out by Phishless, that older parts of plants can show "damage" from gradual changes in their immediate environment such as light deprivation, poor water circulation, etc, due to overgrowth and crowding, as when stem plants get long and leggy, with only the newest growth being healthy. It is not surprising to see the ugly dark algae being fostered by this.