I don't know the answer, but, if you have a repeatable effect, you can find the answer with some experiments. What could slow the glosso down when the Rotala gets thick? One hypothesis would be lack of nutrients (phosphate?) because the Rotala is out competing the glosso. This hypothesis could be tested by giving a big dose of nutrients when the Rotala is thick and seeing if the glosso gets up to speed again. Another hypothesis would be that the Rotala is shading the glosso. The way to check that one would be to make sure that some of the glosso is well illuminated and far away from from the Rotala, even if there is a lot of the Rotala. If it slows down, then we know that the light isn't limiting the growth....... For example, my glosso slows down when my Rotala gets real thick. I would think any phosphate that gets dosed is absorbed by the Rotala, and not much is left for the glosso. When the Rotala gets a hair cut, the glosso picks up speed and gets out of control........