text: More recent variations are built with a simple "waterfall" driven by gravity, using a simple PVC plumbing pipe to flow water down a piece of plastic knitting screen (also known as "plastic canvas"), which is roughed up to allow algae to attach. In almost every case, these homemade algae scrubbers reduced the nutrients to very low levels, and this reduced or eliminated all nuisance algae problems.
  In addition, "turf" algae, which was the focus of Dr. Adey's dumping-bucket design, is replaced by "green hair algae". This is because turf algae tends to be dark brown and thick (like artificial turf on sports fields), and it blocks the light and water from reaching the screen. This slows the growth (and filtering) of the algae because the bottom layers of algae that are attached to the screen start to die and detach. Green algae, however (especially light-green hair algae), allows light and water to penetrate all the way down to the screen if the growth is kept less than 20 mm thick, which allows the algae to grow faster and absorb more nutrients without dying and losing attachment to the screen. This is fortunate because green hair algae is the exact type of algae that grows automatically in a properly constructed algae scrubber.
  Some models also use up-flowing air bubbles. This version, which is basically the exact opposite of the waterfall, allows the algae scrubber to be placed underwater in the aquarium, sump or pond, instead of above it. This greatly simplifies construction, since the device does not need to be waterproof, and it allows placement of the scrubber into tight areas where there is no room above the water line. The design also keeps the algae from drying out in the event of a power failure, because all the algae is under water, and the design also removes almost all splashing. The up-flowing bubble design falls into three categories: those that attach to and shine through the aquarium (or sump) glass; those that float on top of the aquarium, sump or pond water surface; and those that go completely underwater like a submarine.