Filter flow problem
 

I got a ex1200 running in my 55gallon tank, I noticed a few weeks ago the water flow in the return pipe and back into the tank had slowed significantly - it's still flowing alright but no where near as powerful as usual and before. I cleaned out the filter - problem still there though. After a while i decided to clean out the long clear pipes of algae as I thought this may block the flow. I did this along with cleaning the valves and the prop inside the filter head. Turned it on - it was powerful again.

Now 2 weeks later - The same thing has happened, but it just can't be clogged again in 2 weeks! (ive had the tank for nearly a year with this filter and its never been this slow). I don't know what to do now, I can't keep cleaning all the tubes every 2 weeks especially the long ones into the filter as this is alot of work considering I dont even see algae or any clogs in it! Can anyone suggest anything? :?

Algae growing that fast means you have a high level of nutrients. What's your fish load in that tank? Is that your only filter?