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Welcome.

This might help:
http://algaescrubber.net/forums/showthread.php?1291-Algae-Scrubber-Basics-The-Summary

And this:...

Welcome.

The other end of the pipe is either closed, or also connected to the pump. So the only way out is the slot.

It's probably only the light that made the difference.

Cut a few squares out of the screen to make room for the ties.

Nice job. Don't forget you can also just use plastic sandwich wrap to cover the screen.