I want to better understand what points you are making. Why do people have plant health problems when maximising PAR? What sort of health problems? Is it that PAR is increased but CO2 is not increased accordingly?
Yes, that is correct. It's very easy to pummel a tank with photonic energy. Photons move at, well, the speed of light, and they carry high energy into the chlorophyll molecule. The job of the chlorophyll is to absorb the energy and to use this energy as a slingshot for electrons. The electrons are then used to strip CO2 of Carbon and to re-allocate the Carbon into a type of Phosphate sugar. This sugar is what the plant eats, and that's how plants grow - by making their own food.
If you dump too many photons into this system at too fast a rate, without also having an appropriate amount of Carbon to process the electrons then the electrons spew out uncontrollably at a high rate and effectively cause radiation poisoning. The electrons are charged particles and if they are not allocated to Carbon fixing they then combine with any nearby object, often forming something called "Reactive Oxidative Species" such as Ozone and Hydrogen Peroxide. These are deadly to the plant tissue. They can even destroy the Chlorophyll itself. Then the plant is unable even to process the PAR energy and it starves to death. Damaged tissues due to "oxidative stress" appears as translucency, rotting, black spots, brown spots or brown edges. If the oxidative stress is only mild, the damage tissues become fodder for algae, and so we see problems such as hair/filamentous algae, and some Red Algal forms such as BBA.
If there is sufficient CO2, but insufficient nutrient levels then the high PAR causes other types of damage because production of required proteins and enzymes will be throttled. These other types of damage also cause cell death which becomes fodder for other types of algal species.
So there is an entire array of problems encountered when people only think about light, without thinking about the chain of events and the other resources required to process the energy and to turn it into something useful.
Let's simplify this. The amount of light, measured in PAR units, will either be just right, too little or too much.
No. The idea of just right is irrelevant. This is another problem in the hobby where we oversimplify the complicated things and over-complicate the simple things.
There is no such thing as just right. The analogy I prefer use is that of an automobile. The position of your throttle pedal determine the speed of the car. Is there such a thing as an ideal speed? No. Higher speeds consume more resources such as petrol, oil and rubber as a penalty, but the destination is reached more quickly. Lower speeds consume less of the resources, but the destination is arrived at more slowly. "Just right" therefore is only a function of the drivers values, goals and objectives.
The penalties of high PAR are that CO2 must be excellent, nutrition high and maintenance a top priority, but the rewards are higher plant growth rates. Low PAR achieves the same goal but does so more slowly without the consumption of as many resources. Middling PAR achieves a compromise between the two extremes.
The problem is that most people using high PAR have no idea how to also allocate the resources. As I mentioned, getting lots of PAR energy into the tank is easy, but getting sufficient CO2 to the plants is THE single most difficult aspect of the hobby because gasses do not move very efficiently in water. Most tanks therefore have plants that suffer for gas exchange difficulty, and this difficulty is the root of almost all problems.
Therefore, the best approach is to invest in lighting in which you have some control over PAR. In this sense, LED are superior in that most commercial units, although expensive, have a dimming controller. T5 or T8 or Halide lighting can be used but one has to be more careful in selecting the power levels, or one has to also invest in serious CO2 and distribution infrastructure in order to accommodate the uncontrolled higher lighting. There are PAR charts which provide basic guidelines for these types of lighting units. It really is not a difficult task once you are aware of the charts. I refer to the chart in the thread
Dymax Tropical 36 watt | Page 4 | UK Aquatic Plant Society very often and the link has a discussion that is very useful.
Cheers,
