Please note: This is the version that was posted online in early 2012. While the majority of the information presented here is still correct, there are a few things that are outdated. I've been meaning to update this for quite a long time, if you have any questions, you can click the Discussion tab and ask!
There are 3 basic types of light sources that people use: CFL, T5HO, and LED.
Since LED Algae Scrubbers are becoming all the rave, I have further expanded the discussion regarding LEDs. The LED based Algae Scrubber has a different set of rules to follow with respect to wattage and photoperiod, as well as a few other factors. So to avoid confusion, I kept the LED section separate and it follows the fluorescent section.
In the cases of fluorescent lighting, the optimum spectrum / light temp for growing algae is 2700K-3500K, with 2700K-3000K getting the best results.
Proper wattage of light and proper flow to the screen are the critical factors; color temperature / spectrum comes in behind those. You can use higher K ratings, but the real-world (anecdotal) evidence suggests that the optimal range for growing algae is heavy in the red spectrum. If you look at regular plant grow lights, you will find that most of them (especially LED grow lights) are very heavy in red.
Power Compact, or PC lamps, are not recommended, because they run way too hot for the amount of light you get out of them. I don't even care for them for tank lighting.
Quantity of lighting is dependent on the size of the Algae Scrubber screen (dimensional area). In general, you want 1 watt (actual, NOT “incandescent equivalent”) of light per square inch for optimal scrubbing power. You can get away with less, but I would not recommend it. You will likely experience problems at some point.
As you will notice throughout this thread, it is generally stressed to follow the minimum 1 watt per square inch guideline. This is because it solves many Algae Scrubber issues. The reason behind this is scientific. Light interacts with algae and causes N and P (and ammonia & nitrite, among other things) to be absorbed, and chlorophyll is created (among other things). The more light, the more nutrient reduction you get. There is a direct correlation between the quantity of light supplied and the amount of nutrient reduction capability.
A non-vertical Algae Scrubber requires at minimum 1.5 times the light as you would need for a comparable vertical single sided Algae Scrubber. Here is where I will switch to the total surface area method. A vertical single sided 1-cube per day Algae Scrubber would be 24 square inches, lit with 12 watts, or 0.5 watts per square inch of total surface area. A non-vertical Algae Scrubber needs to be 2x as large, or 48 square inches, and have 18 watts of light, which results in 0.375 watts per square inch. I would argue that you need at least 0.5 watts/sq in, if not a full 1.0 watt per square inch to make up for the efficiency loss factor inherent to a non-vertical Algae Scrubber because of channeling and/or water draining through the screen instead of across the algae mat. Older dump-bucket or surge style Algae Scrubbers may not have this problem, but then again, you’re only filtering when the water is moving fast, which is only periodically (again, boundary layer). I hope this convinces you not to try a non-vertical Algae Scrubber, unless you just think it would be cool.
In general, you need to run your lighting for 18 hours on, 6 hours off (see UPDATE below). All life needs downtime. Plants are no exception. They have adapted to the environment over millions of years, and as the saying goes, you can’t fool Mother Nature. So don't go thinking that you can run lights 24/7 and get 25% more algae growth, it doesn’t work that way. The lights should be run on the reverse cycle of your display tank lighting; this assists in maintaining pH at night, as well as spreads the light-induced heat load more evenly throughout the day.
You want the lights as close as possible, within reason. The effective power/intensity of light follows the inverse square law. If you move a light twice as far away, the intensity drops by a factor of 4. If you move it twice as close, you get 4x the intensity. The balance point seems to be about 4" from the screen for CFL, and about 2" with T5HO. The reason for 4" away for CFL stems from hot spot issues due to the concentrated signature of the lamp; CFLs need to be a bit further away to cover the proper area without too much intensity. T5HOs do not have this problem, as the light is very evenly spread.
As far as spacing is concerned, CFLs need to be spaced according to the allowance of the design. If you need 2 per side, just position them for the best coverage. This is really on a case-by-case basis.
The advent of the smaller, higher light screen (discussed in detail to follow) has presented an issue with respect to CFL Algae Scrubbers – how to fit all that light into an even smaller area! There is no rule saying that you cannot trim the CFL reflectors so that they can ‘cross over’ each other. I literally just thought of this as I was editing this. Think of it like when you’re watching a movie and someone is looking through binoculars – you see the “8”-on-it’s-side shaped viewing area. Just trim both reflectors where they intersect using some tin snips or a wire cutter. (Be careful of sharp edges!) You may lose a little light from one lamp that is throw to the adjacent lamp (which it can now “see”) but since we’re concentrating the screen down in size, we need acceptable losses, and this is one. There are also socket splitters that you can use so that you can put 2 CFLs into one socket. However, you lose a little more with this method because the lamps will extend out from the reflector.
As for T5HO, you generally want a lamp spacing of 2-3". For T5HO, your Algae Scrubber will generally need to be designed around the lamps and spacing. CFLs are more flexible in this respect, allowing a variety of configurations.
If you run your lighting 18 on/6 off, the lamps must be replaced every 3 months. This is not just a rule for Algae Scrubbers, you will see many people make this suggestion for refugium lighting as well. That is because there is a power drop-off and a spectrum shift that takes place over time, and when you go much past 3 months, you hit that drop-off point. We can't see the difference, but then again, we're not algae - it can. The result is that your screen will slow down growing and reduce filtration, which you do not want.
The light source needs to be positioned so that it is pointing directly at the screen material. Do not place the fixture so that it points parallel to the screen (from the ends or the top), place it so that directs the light toward the screen. Perfectly perpendicular is optimal, but if you have to point it at somewhat of an angle just to make it work, that will be fine. This is more of a concern for CFL than linear sources (T5HO), however I have seen a few T5HO build with the lamp 4 inches above the screen, shining straight down. They didn't work so well.
Big Basic Change #2 – Alternative High-Intensity Lighting
Around the same time that the screen sizing method was changed, another suggestion was made: you can use twice the wattage of lighting and cut the photoperiod in half. This has a couple of advantages:
The first and primary advantage is the fact that more intense light promotes greener growth faster. This is especially useful in systems that have an initially high nutrient load, or systems that are overrun with algae in the display tank. More intense light will penetrate deeper into the algae mat. In a high nutrient system, the screen tends to grow darker (brown or black). This dark growth prevents the light from reaching the ‘roots’ of the algae, which may lead to the algae detaching from the screen more easily between cleanings. In systems that have a lot of algae in the display tank, it is more difficult to establish a preferred growth location (the Algae Scrubber screen); increasing the light level on the screen can greatly assist in this battle.
The second advantage is increasing lamp life, or more accurately, increasing the time between the need for lamp changes. Since you are only running the lamps half the time, they will last twice as long. This is of course offset by the need for twice as many lamps, but if you combine the new screen sizing guideline together with the higher-light guideline, in most cases you are reducing the overall wattage requirement by half or more versus the old tank-size guideline.
The caveat to this rule is that you must keep an eye on the type of growth. You want green hair algae; anything else requires an adjustment to the ON cycle. If you are getting dark growth, then you need to increase the photoperiod. If you are getting yellow growth, you need to decrease the photoperiod. I suggest making adjustments in 1 or 2 hour increments per week until you get the desired green growth.
Also, as the lamp will eventually start to weaken, you need to pay attention to your growth. Around the 3 month point, the growth may start to get darker. If/when this occurs, you will need to start increasing hours until it grows green again. When you reach 18 hours, it's time for new bulbs. Keeping track of the “on” time that you use will greatly assist you, as you will be able to see a pattern developing. Growth can vary from week to week, but should be relatively consistent on the long term, as long as nothing else changes; changing feeding amount, rearranging the tank, adding more LR, an unknown dead fish, etc, can all affect the color of the growth. Constantly fiddling with the photoperiod should not be necessary after the Algae Scrubber is “dialed in”. You should only need to adjust upward about 1 hour per week as the lamps weaken.
NOTE: Yellow growth is a spongy, gooey type of growth that is also great at light blocking, and is a result of not enough nutrients being delivered to the screen/algae in proportion to the light provided. To resolve this, increase flow if possible, add iron, or reduce the number of hours. You can also increase feeding as long as it’s not enough to overwhelm the Algae Scrubber and cause dark growth.
T5HO Users: This rule has a little bit of a drawback to it for T5HO users. If you are using a stock fixture, this method doesn’t really apply. However, if you are getting yellow rubbery growth, you can back down the photoperiod. However, I would try trimming the screen down first. I did this and the growth got much better. I was running a 20” wide screen and after taking some PAR measurements, I realized that the light in front of the last inch of exposed lamp dropped dramatically, so I just cut that part of the screen off (actually I cut 3” off each side) since I was just getting red turf there anyways and was tired of scrubbing it off. If you are using a DIY T5HO fixture, you might be able to use narrower reflectors and squeeze another lamp in. You lose a little in reflector efficiency, but gain more in additional lamp wattage.