Yes Alex, you are right about the algae - there is a lot of it on the plants
and its bright green (hair like) but again the PO4 is low and the nitrate is
also very low. I suspect that my maximum lighting is not good
enough.....perhaps I should stop CO2 injection as I doubt it is doing any
good at the moment.

I do vacuum the gravel when I perform the water change and I also vacuum the
debris off the plants as well. The next morning its all back. I have stopped
everything now......Gravel cleaning, disturbing the gravel and I am going to
leave it for the weekend and see how bad it gets......Then perhaps another
150L water change on Monday when I will leave the pumps off for a few days
to see what happens.
If it is back with no pumping then it must be something other than just
dirt!

The circulation is twice that recommended for my tank volume - but again the
part of the filter that's taking the water into the canister is small in
comparison and I have noticed (when the tank was not so full of plants) that
the detritus was never sucked into the filter with any force at all - often
it just sailed past it. Though the return jet is quite powerful.

This is all leading me to the conclusion that Eheim filters are crap! and
useless.

My tank is a an Effect Line and there is only one place for in/out tubes and
that's at the top left. So the inlet is at the top about 1/3 way across and
pointing horizontal (under the surface with minimal surface agitation). The
outlet is towards the bottom (at the back and midway in the tank), it is
only about 3" long 1" diameter. Water is moved to hit the far side (right
side) around the side and then back along the front - it moves down in doing
this as well.

Well that's the explanation
Martin