In addition to light and CO2, plants need nutrients to grow. Nutrients are
divided into two categories:
Macro Nutrients: Nitrogen, Phosphate, & Nitrogen
Micro Nutrients: Trace elements (iron, zinc, etc ..)

In the presence of good light and CO2 levels, the plants will grow until one
or more nutrients is expended. At that time, they will stop growing (and
consuming nutrients) even though other nutrients are abundant.

In your case, the limiting nutrient seems to be Nitrogen (nitrate levels are
very low). It is understandable that your plants stopped growing or slowed
down. Unfortunately, unlike plants, algae can fix its own nitrogen and can
therefore continue to grow in the absence of nitrogen as long as the
remaining nutrients are available (all in the presence of light and CO2).

Therefore, what you need to do is make sure that there is ample supply of
nitrogen (as nitrate), potassium and trace elements. Eventually, all of the
available phosphate is consumed by the plants (and algae) and phosphate
becomes the growth limiting factor. At that stage, your plants -being the
dominant macro organisms- will be able to out-compete the algae for the
available phosphate. The algae disappears.

In summary

- Add Nitrate to 5-10 ppm - raise the level slowly over a course of a week
(after you have dealt with the excess phosphate)
- Add Trace elements using a kit like SeaChem Flourish
- Some people suggest the addition of potassium as K2S04 or KCl to make sure
potassium is in ample supply
- Maintain your current levels of light and CO2

Your phosphate levels should always be near zero (0). If growth becomes too
hindered, you can control it using very small doses of phosphate from a
source such as fish food or fleet enema

In the short term, however, you need to reduce your phosphates in order to
arrest the algae bloom using one of the available products on the market.

And, get rid of the carbon filter, it absorbs your micro nutrients (trace
elements) making them the limiting growth factor.

Don't forget the weekly 25% + water changes to dilute any excess nutrients
you've added to ensure they are in ample supply.