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Algae Scrubber Basics 6 of 14

Posted 02/19/2012 at 09:32 AM by Floyd R Turbo

Screen Dimensions

So now that you know your actual flow rate AND the necessary size of your screen (dimensional area), now you are ready to figure out your dimensions.

You want the flow to be a minimum of 35 GPH per inch of screen width. You can get by with a lower flow rate, but your Algae Scrubber may not be strong enough depending on your bio-load. You can have higher flow also, which is generally not a problem as long as your screen is rough enough and you aren’t getting black slime algae (which is a sign of high nutrients, and needs more frequent cleaning until it lightens up). What you want to achieve is enough flow so that you have a full sheet of water across the screen off the bottom edge, like this:

Simply take your GPH (that you just measured) and divide by 35, and this will be your maximum screen width. Then, take that number and divide it into your total screen area to obtain the height dimension of the screen. The result is the total area of roughed-up screen that you want.

You want to add to the height dimension for the section of smooth screen that will be inserted into the slot pipe. You want to allow for the distance that the screen will be inserted into the slot pipe, plus at least 1/8" of smooth screen below the slot tube to help prevent algae growth into the slot. The total amount of smooth screen you need depends on how far you insert the screen into the slot pipe. A little extra smooth screen at the top never hurts, as it can be trimmed off later if you leave too much.

You may also want to leave smooth screen at the bottom as well. Most screens, in some form or another, will ‘dip’ into a pool of water at the bottom. This submerged area does not grow algae very well, so you can leave that smooth too.

The critical area, and the only area that contributes to scrubbing power, is the roughed-up and illuminated portion of the screen. Figure out your necessary screen dimensions, and then add the extra smooth sections to the height dimension.

If the only way you can mount your scrubber is to have it hanging in free air and high above your sump, you can leave a significant amount of smooth screen below the ‘effective’ area, or you can attach another screen to the bottom edge of the main screen. You may end up with some gooey growth on this screen though, so you should avoid it if possible (IMO).


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.
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