I have a lucky bamboo that is growing with another lucky bamboo in a vase with little pebbles. It's about 1.5 years old. It's been growing like crazy! Super healthy until a few weeks ago.

Now, one of the bamboo has started yellow at the base of the cluster of leaves. It has just gotten yellower and yellower. Not sure what I should do. The other plant has one leaf that has started turning yellow, but this looks more like damage/burn that I could possibly just cut off as opposed to what the other plant has.

My theories are:
1) i live in Chicago with south facing windows, I used to think the south facing windows were too much so the plant was usually across the room. perhaps in the winter months with less daylight, they are turning yellow because not enough sun? Though I'm not sure why this would start at the base of the tuft of leaves and not affect all the leaves as well as the other plant.

2) My Brita filter is about ready to need to be replaced. Perhaps the water isn't filtered enough right now if the filter is too old.

3) Maybe it needs to be repotted? The plant that is NOT yellow has grown to be quite tall. Perhaps the other plant that is smaller and hasn't grown as much is being suffocated in the same vase?

I've attached a picture. What do you guys think? Is it salvageable? What should I do?