I am firmly in the long hours +12h at low(er) intensity camp. I want to enjoy my tanks for as many hours as possible during the day and evening. What actual intensity you need to maintain is sort of a moving target depending on plant density as the tank grows in, type of plants, coverage of the light panel and tech level. There are no magic "percentage" - the percentage % depends on the output of LED panel relative to the specifics of the tank - with a very powerful panel you may find 25% being too much... or with a weak panel 100% to be too little. The best measurement we have is PAR which can be hard to manage and interpret - and of course require an expensive bit of equipment. Best approach might be to start somewhere where the light level seems reasonable to your eyes, dial it down a bit, see how the plants reacts and slowly increase or decrease over time.
Cheers,
Michael
Your two points match my experience exactly there. As plant density increases you may need more light, species of plant is also a huge issue.
I think in terms of what looks bright, most folks with no experience underestimate the light they need, many more experienced folks over-estimate. However, George Farmer ran a couple of nano tanks under very high lighting which had him admitting he needed to think again. They worked remarkably well.
I would also make the distinction between the standard light fittings today compared to the relatively recent past (with top rated manufacturing firms) and the needs of red and carpeting, high demand plants.
With my first Juwel tank, admittedly a few decades ago, two fluorescent tubes were factory fitted, I had to buy the rather expensive extra bit of kit then available and run four tubes to grow more challenging plants in hard London water. Move on to T5 tubes, and four tubes were too hot, I have no idea if the light intensity was too much, the four tubes overheated the tank. Come LEDs and the fitted two LED tubes work a treat for most plants.
A tank I bought in the mid 1980s - I know, I'm not a newbie - came with one one inch diameter tube, on a 15 inch rear to front tank. I bought one, then later another, domestic warm white tubes from Woolworths, it transformed the growth in my tank. My first tank, 1960s had one standard tube and again didn't really grow much more than duckweed. My then teenage friend in the 1970s grew Indian Fern by the bucket load, literally, bucket fulls of the stuff, with two full length one inch diameter tubes on a 12 inch front to rear tank. He was the envy of my tropical fish club. He gave Indian Fern away to awe struck adults.
I would normally say, if a tank is less than 18 inches deep, 12 inches rear to front two T8 tubes, 18 inches 3, 24 4 tubes, with T5 tubes less tubes, but never easy to know how many fewer. With LEDs things become so much harder to work out because the Lumen to watt output varies and the cheaper LED bars are often even on a 40-44 inch long bar only 30 or 40 watts, good luck getting anything but algae and maybe some species of crypt to cope with that. Again, only in my experience, I am not a scientist.
A long photo period works a treat with CO2 injection and certain species of plants, some species in my experience however, literally, close-up for bedtime after roughly 8 hours. And, if your tank gets hit with sunlight, typically early morning or late afternoon, having the lights and CO2 kick in will definitely reduce algae issues. But then with no CO2 I found the siesta useful, probably because it balanced out the amount of CO2 in the water column. Irrelevant, if not positively harmful once CO2 is injected.
I use a lot of light, currently 260 watts on a 4 foot tank, it is no algae feast, but sometimes when plant density gets high, I use 310 plus, I time an extra light to come for several hours during the middle of the photoperiod but I also do have to turn up the CO2 to compensate/match the extra intensity.
I think the answer to the original question is not actually, at all simple. And you are right there is no magic percentage. PAR is as you said problematic to measure and I find that colour temperature isn't always quite what I want, too much blue/purple in the spectrum on some aquarium dedicated products, which reminds me of the old Triton T8 tubes, advertised as break through but relatively quickly discontinued. Everybody I knew had algae issues with them, I think they were too blue, I doubt they were much more intense, I don't think the technology allowed for that. And as for the old Grolux tubes, useless on their own but actually quite useful when combined with more intense tubes of daylight spectrum.
I look forward to the views of the more scientifically trained.
