Block Spots in the white grout between tiles

[arranp]

I have black spots on the grout between the tiles. Currrently I use a hookah diving system and toothbrush, it takes hours to do just a small area.

Is there a chemical or an easier way to lift the black spots fromt he white grout.

Thank you
Regards
Arran.

[Teapot1]

Black bacteria, cyanobacteria, incorrectly named black algae by the pool industry is tough to shift as there is a tough outer shell you have to abrade off as you are doing so chlorine can get in. Often the black bacteria will return. Stronger than chlorine is chlorine dioxide which can kill it to a degree. Holding a calcium hypochlorite rather than the weaker sodium hypochlorite may help once you have scrubbed the surface.