Thats a core tenet of the pest algae challenge thread in the algae forum.
cyano and gha variants can show up anywhere, any time, vectored in by the air and common contamination pathways (some cyano and nitrifying bacteria and x amnt of other bacteria are under anyones fingernails who touched garden soil, for example, to varying degrees)
but others like invasive macro algae, valonia, invasive dinos, bryopsis if truly id'd as bryopsis and not the hundred and fifty gha simulants we mis id, and many others are strictly obligate hitchhikers.
The reason this is impactful, is because for obligate hitchhikers nutrient control isn't required to beat them. you can simply export biomass or kill it in creative ways, and if you can get to it all, the tank w be free of it even if you increase phosphates threefold.
GHA and cyano are always part of our biosystems, in one state of repression or succession.
B