Not adding any PO4 is likely your issue. Plants need PO4 as much as they need CO2.
Follow the link in my sig to the nutrient calculater to work out what you need to be adding. For your lighting Id add 60% of what it tells you to, (Tom please correct me if Im wrong on that).
You need to provide everything the plants need, once your plants are growing well, they will outcompete the algae for resources and the algae will go.
Its a commonly stated misconception that PO4 causes algae.. limiting your plants growth by anything other than light is a large cause of algae.. algae is an opportunist beast, itll jump on any gaps in your maintinence/feeding schedules and exploit them.
In your garden outside, what happens if your plants arent growing well? They become stunted and unattractive, but also, weeds take over. Algae is just a weed.