I just had a bad algae outbreak (brown hair algae). Couldn't figure it out - - THEN - I discovered that the ball valve diverting flow to my refugium had been slightly altered, and the flow had really dropped a lot. It is full of caulerpa - and the fact the flow dropped meant that there was enough nutrient for algae - and the caulerpa was getting starved. Phosphate and nitrate stayed zero (eaten by the algae).
Mostly fixed within a week or so of turning back up the fuge flow.
Should have used a gate valve instead of a ball valve.
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