It's been recently discovered that the screen-wrapped-around-pipe design will clog up and stop working when the algae gets thick. It grows heavily where the screen is wrapped at the top, and blocks further flow from going down the screen. Instead the flow starts going out the ends, completely missing the entire screen, thus killing what algae was previously growing on screen. It affects different screens in varying amounts; the higher the lights are (thus closer to the wrap), the more it clogs. This is the type I'm referring to: