I have had similar problems with algae in my Turtle tank. I have two Fluval 305 filters (those terrapins are VERY messy) and I found when I have an algae explosion the cause is more often restrictions in the intake hose caused from algae growing
inside the hose, rather than problems with the media.
I now check the outtake for low flow once every three days or so by pulling the hose out of the water and seeing if it is merely a little dribble. Merely forcing water through the hoses does not get rid of the algae in the hose.
I recently found the ULTIMATE tool for cleaning the hoses. It is a 3' "Drain Cleaner" I got the one I have from
Harbor Freight,
but you can probably get something just like it in any local hardware store. A 4' model would have been better for my setup but this thing works like a charm!