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[APD] light and the w/gal rule
Watt per gallon has worked very well for me over a number of years, decades I suppose now.
On some nano tanks, I do tend to add more, but I also suspend the lights higher.....so it likely is about the same intensity. Few folks have tanks over 24-28" deep.
So depth is really not an issue(at least as someone who has set up and maintained more 24"+ deep tanks than I can honestly count).
Folks want to do all sorts of math, that's fine...........but you do not get any good measurement of light that way, nor with Lumens, lux, and what have you.
Micro moles of photons/m^2/sec is the best unit overall for plant biology.
These run about 550-2000$, not something hobbyist care to indulge in.
But since we add CO2/nutrients to excess, the main thing we limit is really light.
That _is_ the throttle. It has a limit, perhaps 500-600 micro moles or so for most aquatic plants. This is about 1/4 of full tropical sun.
Overall, there is no precise method to measure light for aquarist.
So a range of watts/gal is useful and for all the banter over the rule being bad etc, I've not found it to be any issue at all.
You can use less or more, but the middle range is good and you can grow pretty much anything you want in every tank except some very extreme cases.
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