1. I'd suspect overfeeding too. Too much leftover food = ammonia. Ammonia not taken up fast enough = algae.
2. I'd do a big cleanup... the blackout will not be good for your plant mass (see below).
3. You've probably not included enough plant mass (subjective), not good enough CO2 circulation throughout the water.
Get fresh new plants in there while cleaning up as best as you can. Your filter probably needs a wash too from the sound of things.
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