[ChrisX]

Honestly, it just looks like they are slow growing and attracting algea on the older stems.

Most people top these and space them closely so they grow in a colorful bush.

As they get close to the light in your tank, the tops get more PAR and develop color. If you trim them short, they will get much less PAR and grow slowly. This is the nature of this plant, eventually after topping them several times, you have to start over because the original stem gets bad.

It looks to me like you don't have enough light for this plant, and they should be topped frequently to keep them in a bush so you are only seeing the exterior leaves.

The lower parts of the stems are probably many months old and if the tank is not 100% algae free, the old stems will attract algae.

I am keeping rotala with some success, but the old stems will eventually get this way. The solution? Top them frequently, plant them close, and run high light so they are always pushing out colorful new leaves on top.

 

[ChrisX]

Can you clarify what you mean by topping in this context? I've always understood this to mean cutting the tops and replanting them in place of the bottoms. But in my experience, replanting the tops prevents them from growing bushier, since additional nodes aren't generated from the clipping point of the original stem and the new growth on the topped (replanting trimming) stem is less bushy.

By topping, I mean just trim the tops a few inches below where you want the ultimate height to be.

Where you trim, it often will send out multiple new stems so it will be thick.

When the lower stems are bad like that, remove the old stems and just replant the tops.

How to grow Rotala Rotundifolia

The attractive orange-red Rotala rotundifola is a fast growing stem plant that has been a staple of the planted aquarium world for a long time. This is a detailed guide on how to grow Rotala Rotundifolia, including how to overcome challenges in keeping this species.

www.2hraquarist.com

IMO, I am just guessing here, you don't have enough PAR at the substrate to get good growth down there, so you let them grow tall (they will search for the light), then they start to color up, but you are left with the ugly stems.