What about all the fear mongers claiming testing PO4 and NO3 are so important for planted tanks?
Then they do not test light:werd:?????????
Light meters cost $$, but are fairly easy to use.
Testing PO4 and NO3 is relatively cheap.
Doing NO3/PO4 testing correctly is more involved and rarely done.
CO2 is the lynch pin for those that use it.
It is tested even worse and there are many issues there, but cost, scale, time etc.........
Still, comparatively, PAR is about the best parameter can can use to measure light's effect on plants. By measuring several nice well scaped examples as references, we can begin to get a range that is useful for any species that is grown in such tanks.
Since we have a control or a reference aquarium, the rest of the problems MUST be due to nutrients or CO2 since we know based on the examples from the nice tank references, the light is independent and not a factor in the lack of success.
Nutrients are easy to rule out as a limiting factor and the aquarist does not need any test kit to do this with ferts and water change via EI.
This leaves CO2 as the main factor.
But........CO2 is tricky and involves current, O2, different species, filtration and flow through, type and flow rates through the diffusion method etc.
Many get impatient with CO2, and kill their fish, no one has killed fish near as anyone can tell from overdoing KNO3. But the fear mongers seem to conveniently ignore this.
Still, less light = less CO2 demand, so easier management.
30-50 micromols is ideal for 90% or more of the aquarist goals, it also uses much less electricity and saves a lot on the electric bill.
A PAR meter can be bought and shared among a group of aquarist, or bought, used and then sold. We use a club meter and share it. It does not get a lot of use, it's a fast thing to test and measure, not hard.
Main issue is the cost. It is hard to say how much light folks have, there are plenty of surprises. Most think they have less than they do, ADA was the other way around, much less than one might predict.
T5's are really quite bright, about 200-300% more than the old Watt/gallon rules that used T12's 2 decades ago. 200-300% error is quite large, not calibrating test kits can also result in similar ranges.
Ironically, some have criticized EI dosing for being inaccurate/wasteful, but compared to what? Light? Co2? These two cost more and pose far far more risk to issues with planted tank management than anything to do with ferts(which are the easiest things we can control).
If you subscribe to "less is better", then it has to start with lighting.
What is just enough light?
Regards,
Tom Barr
if someone has something i haven't listed i'd be happy to give it a thrice-over. 
