I've had people argue with me about SMD LEDs vs high power PCB mounted LEDs and at this point, here's where I stand with that:
Low power and SMD LEDs do work to grow algae. The issue becomes, how well do they work in the long run? And I'm not referring to length of life, I'm referring to stages of growth of the scrubber.
A low power LED, if they are packed together for good coverage, will cause a new screen to grow algae, now problem. The problem is, from what I am starting to see, that once you reach a certain thickness of growth, the lower power LEDs don't have enough "punch" to get light through the thick growth to the base of the screen where the algae attaches. The result is, die off. So you have to clean the screen when it gets to a thickness that is enough to cause this. This is a bit of trial and error based on your screen and your system, so it's hard to say "X day when using Y LED for Z hours".
A high power LED will have enough "punch" to keep the base growth alive longer, in most cases. The longer you can let a screen grow without die-off, the stronger the scrubber will be over time (long run), because you have a large mat of algae that is growing at every level. When you remove all that algae, yes it grows back but the uptake is likely to be much less for the next few days after cleaning vs what it would have been could you have let it grow for 2 more days.
So here's where the trick comes in. You want a lower power array for starting the screen up, and then when you get to the point where it grows thick enough to self-shade, punch that light intensity up. Around that same point in time is when your screen reaches "maturity", when a good amount of the holes are filled in after you scrape it. That leftover algae is capable of accepting the higher intensity light, and it can grow back fast. Until the screen gets to that point, higher intensity light can easily result in bare spots on the screen (photosaturation).
So the bottom line here is that it is my opinion that lower power LEDs will only be effective at growing algae to a certain point. After that, you need high intensity light to get growth that can stay attached for a longer growth cycle.
Not saying that you can't use lower power LEDs. You certainly can, but there will likely be a limitation on thickness of growth you can achieve compared to high power LEDs.
There are also caveats to all of this, such as an ultra-low nutrient tank, where you just don't have the nutrients to support much algae growth. I've also seen instances where screens seem to be growing fine and then they suddenly turn start white (which I call "whiting")...can't explain that one yet.
