with the 2 wpg light level and DIY CO2, you will not have a high growth rate which requires daily fertilization. You are right that you may have enough phosphate from the fish food and nitrate from the fish themself, but could lack other items like potassium and trace.
I would not add nitrate if you have a steady 5 or more. That level would be sufficient for your CO2 and lighting.
I think you might want to consider dosing an all-round fertilizer like Flourish once-twice a week to help replace what the plants pull that is not generated.
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