Why would strong flow cause this type of algae? That seem counter intuitive to everything I have read.
For the same reason BBA loves to grow on filter outflows. Especially if that outflow is the source of diffused CO2. Yes, it's 100% counter intuitive. And I can't explain why it happens, nor can anybody else. But it does happen.
its not co2 related IMO, it has something to do with the Ca and Mg, is your Ca level very low? or dosing too much Mg?
That is the explanation offered by the MCI algae diagnostic guide. Though I don't use MCI, I'd always found it interesting how specific the diagnostic portion is. This nutrient=this algae. I'd never had a problem with staghorn before, and I'd recently changed Ca/Mg levels. So reverting my Ca/Mg dosage what it was previously was the very first thing I tried.
It had no effect, at least in my case. Neither did adjusting ratios/levels according to MCI. Or any other combination of changes. And I have almost total control over those two nutrients since my tapwater has a GH of only one.
From there I went on to try increases and reductions of every
nutrient. Three months of tests, not a single improvement in the staghorn. From there I went on to look at light, CO2, and flow, and that's when I started understanding this algae.
Too much light with too little flow and/or CO2=staghorn
Too much flow and/or CO2=staghorn
Too much light, flow, and CO2=LOTS OF STAGHORN