Guy, said - "I believe this because many organisms like to eat the new growth but almost nothing will eat the older growth." Could this be because the newer growth is more tender? We prefer to eat many vegetables just as they ripen not after they have been ripe for a week or so for this same reason - more tender and more flavour.
The "pollution" you talk about. Is the nitrates and phosphates? To plants that is their food - what they use to grow. Are you talking about the toxins that some caulpera excrete to slow down/prevent the growth of other plants in their area?
I'm just trying to figure out exactly what you mean, Guy.
I agree that macro algae needs room to grow and you should harvest regularily to give it room to grow. A fast growing macro algae will consume more nitrates and phosphates than a slow growing one. Older growth does usually grow slower than new growth.
Algae needs two things to grow - food and light. It will only grow until one of it's required foods is depleted (this is the limiting factor). Billsreef's suggestion of iron is good as often iron is low in our tanks. Poor light is often the case (especially if you are using NO bulbs). NO bulbs don't have the intensity to light more than the top of the algae. PC bulbs and even your old MH (which some people use) provide a lot more intensitiy and more growth.
FWIW I have my algae grow out tank (tang heaven and ulva for tang food) on a window sill. I found that the refuge with mangroves, grape caulpera and another caulpera with my sump with chaeto was not growing nearly fast enough for the tang so I removed the tang heaven and ulva into a special tank just to grow for food. Since I have low levels of nitrates and phosphates, I think the limiting factor is the light but at the present time I can't change that.
You quite often test 0 for nitrates and phosphates because the macro algae has removed them from the water. This is what we want, but without extra nitrates and phosphates, the algae can't grow. This is why the suggestion of feeding the tank more was a good idea. Pruning also does the same thing by removing some of the algae, then there is excess nutrients again which stimulates the plants to grow.
Vickie