Some people have small spots on the screen, about 1" (25mm) wide, that have no algae; these spots are scattered across the screen (not just near the bulb). These spots are where algae actually WAS growing, but the algae could not hold on, so it let go and went into the water. The reasons the algae could not hold on are:

1. The screen is too smooth (most common problem). No matter what material you use, you should use rough sandpaper to really mess up the surface. If the material is clear (like acrylic; not recommended), you should not be able to see through it at all. If the material is not clear, you should not be able to see a reflection, at all. If the material is a solid sheet, holes should be drilled every 1/4" (6mm)... instead of every inch like many people have tried. With solid sheets, instead of drilling holes, it's better to lay a layer of rug canvas, plasic canvas, shade cloth, or perforated drawer liner, across the surface. You would sand this also.

2. The screen is too thin. Screens should be about 1/4" (6mm) thick. This is thicker than most materials, so you should use two or three pancaked layers of material. This gives the "roots" of the algae more to grab onto.

3. The lights are not being turned off each night (18 hours ON, 6 hours OFF). So the algae grows, but then gets weak because it cannot rest. So it lets go.

4. The flow is too low (the opposite of what you would think). Higher flow delivers more nutrients to the algae (so it can grow strong), and also gives the algae more protection from the light (since the thickness of the water on the screen is more.)

5. The bulbs are actually TOO near, or TOO strong (this is very rare, however). It seems that bulbs that are 60 watt CFL (actual, not equivalent), AND which are 2 inches (5cm) from the screen or less, start to do this. If your flow is strong, then try decreasing the light by either (1) reducing the ON hours, (2) moving the light out to 4", or (3) getting smaller bulbs. But only try one technique at a time.