What I did with real bad algae/ cyano, I found out 2 of 3 of my rocks were leaching phosphates, my nitrates were always low and phosphates always a bit high. I got a hold of the Quantum phosphate remover, ran that for a few months, the science behind that is the Lanthanum chloride or something like that and it binds the phosphate. So it pretty much pulls it out of the rocks and maybe even the algae??
Anyway after running it for a while if you're actually exporting enough from your general maintenance and just need to clear up an old backlog. That's what I found worked for me