The guidelines / rules of thumb the I used and mention below are taken directly from the below link:

I thought I would document my second & first attempt at building a vertical algae scrubber. I upgraded my sump changing the scrubber before I even got my first build up and running. I’m taking a little different approach by trying to keep everything as low cost and low energy consumption as possible. I used the below rules of thumb which are explained in a lot more detail in the link I posted above. Whenever possible, I followed that information as close as I could

Some Scrubber Guidelines that I used:

Sizing of the screen:
If you will have the screen lit from both sides then the size (area) of the screen should be 1 square each per gallon of your display tank (double that if the screen if only lit from one side). In my case, for a 180 gallon tank, the screen needs to be at least 180 square inches as I am lighting it from both sides. As I am also running a skimmer and have a macro algae compartment in the sump, I choose a little smaller screen size of 12 X 10 inches just to make this build easier for me and to be able to use some cheap LED lighting that I already had. On a system without a skimmer, this screen would only be able to support a maximum of a 100 to 120 gallon tank (depending on how it is set-up). The screen dose go a lot longer than that, but the extra material is not lit so I do not count it.

Flow through the scrubber:
The rule of thumb is 35 GPH for each inch of screen with. As my screen is 12 inches wide, I would need 420 gph of flow. This will be plumbed this off my return line which will put the flow closer to 550 gph. From what I have read, more is better.

For a double sided (lit on both sides) vertical scrubber, you should have 1 watt per gallon of water with a color temp of 2,700 to 3,500K for best results. This rule of thumb applies to CFL or T5HO lighting and refers to actual watts, not the “incandescent equivalent”. I could not find anything credible to use for LED or other forms of lighting. I will be using two cheap/knock-off LED panels with a total of 240 watts of LED lighting somewhere closer to 6000K range of color temp. The color temp is just my guess based on my visual observations and comparison to the 6,700K lighting on my fresh water planted tank. I do not know for certain what the actual color temp is making this will be a little bit of a test/experiment for me.

From what I have read, with both higher than required flow levels and higher than required lighting levels, the color spectrum of the light becomes less important. To quote the link posted above, “flow and light intensity trumps spectrum”. I’m counting on that here in my build.

And now for the actual build

As I will have about 550 GPH of flow, I used a combination of ½ and ¾ inch pipe and fittings for the build. I could have just used ½ inch, but seeing as how I had a good mixture of both sitting in the garage, I decided to use them. As long as the smallest sized fitting/pipe can handle the maximum planned flow that will be alright (and it will in this case as well). From what I have read, cutting the slot for the screen to slide into is the hardest part of the build, and they were correct (even more-so since I can only use my left hand). I tried using my dremel with the correct cutting blade but I could not cut a straight 1/8 inch wide slot to save my life. Then the “red-neck” in me remembers the blade on my table saw was also 1/8 inch thick. I must say, cleaning plastic bits off the table saw blade was a lot easier than trying to cut this by hand. I would not recommend doing this very often as it is hard on saw blades

The Risks:
-The lighting that I am using is not the best suited for a scrubber and may need to be up-graded. I am assuming the higher intensity of the lighting will make up for the higher color spectrum.
-The scrubber is plumbed into the return line from the sump, which is after the live rock/macro algae and skimmer compartments of the sump. As they will both take nutrients out of the water before reaching the scrubber, the algae growth might be slower.
-My nitrates are currently around 1 ppm. It may take a while before the fish get close to their full sizes and increase the bio-load enough for a good amount of nitrates the algae can use to grow.