You seem to have a green water at the beginning stage. It thrives in the a
excess of phosphates of water and high light levels (above 3wpg) of light in
the newly established tank. Once the green water develops, the nitrate
levels drop to zero, so the plants cannot outcompete gw and consume all the
phosphates since the nitrates are limiting factor for them. Once you add
nitrates, the plants can now consume the phosphates and the gw for which the
phosphates are essential dies out. However you seem to have phosphates
coming back later on. Do you use any non-aquatic plant fertilizer that has
phosphates in it? or may be you buffering the water for phosphates?