I find this very interesting! I have a Walstad tank that has several VERY robust pothos and syngoniums growing out of it. It is filterless with only a powerhead for movement. I NEVER have nitrates...always zero (and yes, I shake the hell out of bottle #2). And I too am plagued with Cyanobacteria! To the point where I have to treat with erythromycin, which I hate to do. Should I add supplemental nitrogen even though there is soil?Cyanobacteria is a nitrogen fixer. You usually see them in new tanks with no nitrogen.
I think it's pretty easy to get rid for you. Remove the floating plants and wash off the BGA, then place the plants back.
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