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 While the majority of the information presented here is still correct, there are a few things that are outdated.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!
The search for the "perfect material" for growing algae in the vertical waterfall configuration has always come back to the same material, the Clear #7 Mesh Plastic Canvas. This can be found at almost any fabric (Jo-Ann Fabric) or craft-type hobby store (Michaels), usually in the knitting materials section. I have seen it at Wal-Mart, next to all the yarn. If you google “plastic canvas” you’ll find hundreds of links. You can feel free to experiment, but this material has been proven to work over and over again.
The reason that it works so well is because it is cheap, flexible, light, and easy to rough up. It is also translucent, which is critical. As algae grow thicker on a screen, the outer layer starts to block light to the lower layers. This causes weakening of the algae at the point of adhesion to the screen material. Allowing light to penetrate to the base of the screen from both sides helps to prevent the lower layers from weakening and detaching from the screen. Any material that blocks light is fundamentally inferior, all other things being equal. This also means don’t use any of the colored screening materials that you will see.
A brand new screen is slick and smooth. Except for the top edge of the screen, it needs to be roughed up so that the algae has anchoring points. This is a critical step and it absolutely must be done properly. Once the screen is established, algae can grow very thick, it can hold a significant amount of water, and it will get heavy. I have seen pictures of screens that weigh several pounds. This weight is distributed across the screen, and with a rough base, it will hold very well.
To rough the screen up, the best thing to use is a bi-metal hole saw. These saws have teeth that protrude out to a sharp point, and are great for tearing up the screen
As the above picture shows, you need to drag is back and forth across the screen. I suggest you sit in the garage or outside with a cutting board on your lap and lay the canvas on that. When you start roughing the screen, do a lot of random movements. Don’t concentrate on one spot for very long or you’ll wear through the screen and tear it. Rotate the screen often. You can also tilt the saw bit as you drag it so that the teeth won’t snag the screen. You should get a good heaping tablespoon of plastic from each side of a 10x10 screen.
After it’s all done, your screen should look like this:
Give the screen a good rinse and a light scrub with an old toothbrush to clear off all the pieces that are ‘hanging by a string’. These pieces are inert plastic, but still, you don’t really want these floating around in your aquarium.
Though I haven’t used one myself, I have heard that a rasp (available at any hardware store) is also great for roughing up this material. There are handheld rasps (similar to a Ped-Egg for your feet, except much sharper), and there are bit-mounted rasps (for power drills). It looks like it could be a little easier to tear the screen with this tool, but there is much less elbow grease involved. The drill bit rasps I have seen don’t appear to have protrusions large enough to “dig” into the screen and result in a cactus-rough surface.
Some have tried other methods of roughing up the screen. 80 grit sandpaper will definitely rough up the screen, but it doesn’t leave it ‘prickly’, so it’s not very effective. Recently, someone put a screen in the oven for a few minutes, then sprinkled it with aragonite sand and pressed it on. It was a neat idea, but it blocked the light from penetrating to the base from both sides, so it didn’t do very well.
The bottom line here is that there really is no shortcut. You only have to rough up the screen once, so stick to the tried and true method, unless you feel like experimenting on your aquarium.
As previously mentioned, the top edge of the screen (where it is inserted into the slot pipe) should not be roughed up at all, or else algae can easily grow into the slot and restrict the flow. The amount of material you leave smooth depends on your design, mainly, how far you plan on inserting the screen into the slot pipe. Regardless of this measurement, the smooth section should extend at least 1/8” down below the slot.
Most people have uses standard zip-ties to secure the screen to the slot pipe, since they are cheap and easy to use. Generally you have to use new ones each time you clean the screen (because you have to cut them), or you can re-use them if you’re handy with the tip of a razor blade and can get them to release.
You can also use releasable zip ties (I found them in the electrical section of Home Depot), but they’re not as easy to release as you would think they would be, unfortunately. I have used them, and I still need a mini-screwdriver to take them off.
Velcro straps can also be used, however they need to be free of adhesives, and I have never been able to get an answer from manufacturers about the stuff that has hooks on one side and loops on the other (such as OneWrap) regarding their safeness in aquaria. “Medical Grade” would be the safe way to go, but I haven’t looked much into those.
Another method of holding the screen in place is to use plastic shower curtain rings. It works very well, however you should have extras on hand in case one breaks when you’re removing it.
Yet another is to use sections of PVC pipe, one size larger that the slot tube, to make rotatable rings. This works very well, just make them narrow so that they don’t block too much flow.
The best one that I have found to date is the nylon beaded cable tie, which, like the zip-tie, is inert, but super-easy to release, so it’s good for months and possibly years. I haven’t seen these in any store, I had to order them online.
The ones I bought are perfectly sized to fit around a 1” PVC Schedule 40 pipe. These are my personal favorite choice. If you hang the screen only from the center (with the screen touching the top inside of the pipe) then these might not work very well. Instead, use them at the corners of the screen. Another nice this is that the beads are just a little bigger than the #7 mesh screen holes, so you can just push them through very easily.
TIP: This is an important point – if you have a screen or screens that are held in place in the center of the pipe (or anywhere away from the ends) you do not want to cinch cable ties down hard! Since there is a slot cut in the pipe, cinching down the ties will cause the pipe to pinch closed and you will get reduced flow in the center. The zip-ties should just be snug enough to hold the screen in place.
I’ve seen a few designs that don’t use fasteners, but there are considerations to make with these designs.
Addition on 10/28/15
Over the last few years since I wrote this, it's not often that a new idea pops up that make it worth putting out an update on methodology. Recent, LED lighting has been about the only thing worth updating.
Leave it to Paul B!
Here's a new method for making an algae scrubber screen that, IMO, could very well be a game changer.
It's pretty simple. Smear the screen with a thin coating of a standard mortar mix (like you use for building a brick wall) and then sandwich it between 2 piece of wax paper, and place a wet towel over it. A couple times/day, take the towel and wax paper off and spray it with water. Keep doing this for a few days, 2 or 3. The point is to not let the mortar dry, because you want it to cure instead.
Then, place the screen in water for a few days or a week or something, to soak out the lime and anything else. You're not using much mortar here so it's not a ton of stuff you have to worry about, it's just an extra step.
Now you're good to go. According to Paul, algae loves mortar. Algae does not have this love affair with plastic, which is why the plastic canvas (in most cases) takes a while to get started growing algae thick and well attached: the material has to "slime up" and then build up a layer of calcification or another material so that the algae can attach well.
So what happens is that algae will grow very quickly from the mortar. The mortar is not really permanent though, it will flake off as you clean the screen, but this is not relevant. The plastic canvas still has to go through the curing/maturing process for long-term growth, but while this is happening, you get good growth (and filtration!) from the mortar. Over time, growth will transition to the plastic canvas.
Now, my recommendation for preparing the screen is simple - it's what I do now for my "first stage" of roughing up. I use a wire brush drill bit (the center-crimp kind) and a good drill, running at a low to medium-low speed. I run the bit back and forth with only light pressure (I'm not pushing down and smashing the bit into the screen). I go left-right, then up-down, then diagonal, then the other diagonal. Flip the screen and repeat. That should be good enough for mortar now.
The 2nd stage of roughing that I do involves using a saw blade in small overlapping circles. I'm not sure this is necessary anymore, or maybe it's not just as critical.
I know what I'm doing this weekend...