Using LYE to get rid of Black Mold

[Charisma]

I bought a used 3 person, 180 gallons?, Charisma spa in 2000,  that was probably ten years old. I used it for about 15 years using bleach to maintain it with minor problems with scale which I would rub off. I am the only user of it. I stopped using it for a few years and drained it. Recently I started using it again and now have a black mold problem. The mold seems to be in the bubble jet system as it starts growing out of those tiny holes. I suspect it is in all the pipes also. There is also a small amount of tiny spots of brown mold and the problem with the calcium? scale. These are minor compared to the black mold.

The mold is also in the many tiny cracks in the jell coat. This mold is very tough to remove with a plastic scouring pad. It also came back within one week even after adding one cup of bleach to the tub and no use.

I tried Leisure Time Jet Clean and left that in for 2 days. It had no effect on the mold. I drained the tub and put in 121 ounces of 8 percent bleach and left that in for 2 days and that did not remove the mold. Will LYE remove the black mold? How much would I use?  Any other suggestions? The hot tub is inside a sun room. The picture shows what came back in only week. Thanks for your help.