Black tile joint on a swimming pool.? I have an 8mx4m below ground swimming pool and contained by past...

I have an 8mx4m below ground swimming pool and contained by past years a localised build-up of black occur on some of the grouted joints between the tiles. This year have been in particular bad presumably because of the wetter weather and we presently have profusely of blackened joints. I own presumed that this was a build-up of algae but we do use triple accomplishment tablets all year round which contains anti algae. I enjoy tried concentrated chlorine chrystals and hard brushing but to no avail. Can you support how I eradicate this blackening?
I want 2 run gas pipe out side...
Answers:    Check with a pool chemical store not connected beside a pool service. They can advise you. Take a clammy scraping beside you.

No matter what it is it sounds approaching there will be like mad of elbow grease involved.

Good luck.