Many problems.
Without even asking how much you feed, I can tell from the diatoms on the back wall that filtering is really low.
1. Biggest problem is the light is too small, too far away, and not pointed at the center of the screen. Plus it looks blue (should look warm reddish). You are losing 3/4 of the little wattage that there is.
2. It's a 1-sided horizontal screen that is letting water fall through the screen, so it's not even as effective as a horizontal of the same size.
the holes on the screen is so small that the water only run over the screen
Fixes, from easy to hard:
1. Put a sheet of white acrylic or plastic under the screen, to hold it flat, and to keep the water from running through the screen, and to keep it close to the light.
2. Put flexible reflectors inside the box, to keep the light in. (
http://www.santa-monica.cc/Flexible-Reflector_p_15.html)
3. Find a lower "K" bulb... 2700 or 3000K is best.
it,s a 3500k
4. Get a 45 watt bulb instead.
5. Rough the screen up a lot more, on both sides.
6. Rebuilt it, 2-sided.