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This is exactly what I have been saying hypo works for longer in the tests I did on my own properties and gave instant results unlike bio that takes weeks or months to work and there is no residual preventative with it as they claim .Didn't you apply biocide to your roof a while ago? What brand did you use as it doesn't sound like it's very effective?
Maybe softwash half and biocide other half and see how it fares in 12 months?