We were having algal problems in one of our ponds, the nitrates were fine, no ammonia, pH good, what was going on??
Phosphates! They were sky high!
Phosphates accumulate slowly in fish ponds and aquariums, and the good bacteria doesn't get rid of it! It's present in fish foods, fish and plants.
If phosphates are high, do several BIG water changes to reduce it and don't forget to use chlorine neutraliser!