One typical complaint about aquatic planted aquariums by both the seasoned experienced aquarist as well as the newbie is the rate of fast growth of aquatic plants and algae. How might we control this rate of growth logically based on how plants grow? Most want good healthy growth, but just not this fast!!! So what tools do we have available to manage the rate of growth, that also helps us manage the rate of growth of algae as well? Light intensity of course.
This concept has been around for some time (but has not been really looked at critically) and states: non limiting CO2 and nutrient allow for the maximum light use efficiency at lower light intensity. Since if the CO2 and nutrients are non limiting, thus independent of influencing any reduction of growth, light will be the only factor that will control growth. This simple test is able to look at light's effects on aquatic plant growth much better than dependencies typical with many aquarists' planted aquariums that might be limiting nutrients or CO2.
Light is much more stable than CO2 or any nutrients: we can control light very easily, CO2 ppms and nutrient concentrations can move around considerably. Light is very stable on the other hand. Bulbs can be added or reduced, timed, metal screen can be added to reduce intensity to suit, PAR light meters are very simple and easy to use and testing is rarely done more than once every few years for bulb decay. Open top and suspension style lights can be adjusted to adjust the light intensity to suit any growth management desire/goal the aquarist might have.
Tropica's web site has a good effective article on the use of light and CO2 management for aquarist.
They also suggest lower light and good CO2 in their conclusion.
CO2 can also be used to reduced growth as well, but this limitation is harder to measure and control without going to a pure non CO2 addition method. Many plant species are poor competitors for CO2 and do poorly in mix communities, thus we cannot keep as many species using that method.
This is large trade off if we chose CO2 to reduce rates of growth.
Likewise, some plant species do better at lower nutrients than others.
However, most plants are fairly similar to their lower ranges of light above a certain minimum level.
This appears to be about 30micromoles/m^2/sec, there are plants that can still grow at less than 1/2 this amount, but most will do well and grow slow at 30. A range of 30-50 is suitable for most systems for slow, very easy to manage growth.
This also reduces algae growth since they are only limited by light and never nutrients or CO2 in planted tanks.
It also reduces the amount of energy required to grow plants, initial cost of adding more and more lighting.
This increase in extra light cost aquarist a lot of $ over time.
If a kW/hr is 12 cents, and the light for a 100 gallon tank is say 2x what is required to achieve 50 micromols, say 2w/gal compared to 4 w/gal, and the total is 200W vs 400 W, then 200 W x 10 hour day X 365 days a year = ~88$ a year of wasted energy, algae issues, issues with CO2 that are typically not an issue etc.
Some aquarist want to garden more during sometimes of the year or when their motivation is high, some want to reduce it due to other aspects of their life, vacations etc. If we use a bank of several light bulbs on different switches, we can adjust the intensity to suit most any goal. Many aquarist know they want nice growth that's healthy for their goal, but not weedy fast growing plants they have to prune often.
Low light is ideally suited for that goal.
Since we know light drives CO2 uptake, which drives nutrient uptake, using less light provides far more wiggle room and resiliency to management of both O2 and nutrients. Everything is much easier to care for.
This makes any goal of sustainability and stability much better. This is based on the holistic model of how a plant grows.
I would suggest aquarist try to manage a tank and slowly reduce their light intensity down.
A meter may be borrowed in many cases to target the lower ranges we know are effective. While we may be able to go even lower, 40 micromoles seems like a good range with enough cushion on the lower end and still without proving difficult or producing weedy growth.