The problem is that these plants are not grown initially in an underwater environment. They have access to 400ppm of carbon dioxide from the surrounding air. As a result their leaves are often thicker with a waxy cuticle to protect themselves from pests and the elements.
When you put them under water the co2
availability is cut off. In order for the plants to maximise their ability to scavenge co2
under water they must produce new leaves. They are often thinner and a completely different shape to the leaves grown above water. The plant must then work on production of an enzyme called RuBisCo. I donít want to get too much more in to it unless people are interested but basically the old leaves are completely useless to the plant now that it is unnaturally placed underwater. So they are discarded in favour of new.
When this happens the leaves begin to break down and release substances that algae take advantage of. This is why it is so important to focus on new leaves and why it is imperative that co2
availability is STABLE. If the plant senses altering co2
availability in the water it will discard old growth for new leaves accustomed to the new co2
level. And the algae cycle continues. The first attempt at new leaves can often appear small, pale and stunted giving the impression of a nutrient deficiency.
In my opinion It is better to remove the old leaves when you can see new growth.