Join Date: Feb 2009
Location: Los Angeles, CA
It seems that there is a general consensus when it comes to algae (someone please correct me if I am understanding this incorrectly).
If you have high light, you need two things. First, CO2, and second, a high plant density.
The reason for the first is that algae is more adaptable to/tolerant of low CO2 concentrations, and can thrive in a tank even when the plants are starving. The reason for the second is that enough plants (when given enough CO2) can somehow "outcompete" the algae, and you can have a (relatively) algae-free aquarium.
What's bothering me is why this would be the case. Not so much the CO2 part - that's easy enough to understand. But the high plant density part.
Exactly what are these plants outcompeting the algae for? Not CO2, we've already established that the algae is better adapted to low CO2 levels than the plants. It's not light, either, as algae can live either in the water column itself or on the leaves of the plants, and therefore can receive light.
There must be some other nutrient that algae needs that the plants use more rapidly - and can better survive without. Whatever that nutrient is, it must not be necessary for fish, as fish do just fine in heavily planted tanks.
What is it? Nitrogen? Potassium? Something else?