Caz,

If you have algae then you do have excess P or N, or not enough CO2.
Do not trust the test kits, use them but watch and trust the plants
more. The plants (and algae) will tell you if something is wrong.

A tank with P and N that are indeed close to zero (say a heavily
planted, well lit tank, with a few fish) does NOT have algae.

The best approach is to not try to fight the algae but to make sure
the plants are bubbling and growing. If they do it is only a matter
of time for the tank to clear up from algae.


--Nikolay