More than likely just too much light for your setup. NPK dosed doesn't cause algae, so there's no reason to be afraid to provide K and P. Healthy plants usually don't have algae issues, but if growth is slow with a lot of light it can happen. Co2 increases growth tremendously so it can be the difference between algae getting a foothold or not. Without co2 all you could do is shorten light cycle, increase water changes and use organic removal products like carbon and/or purigen.
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