Black marks on fibreglass pool

[AussieSwimmer]

Hey guys, I've been getting these black marks on my pool and if I rub a vitamin C tablet on them they go away - I told this to an expert and they still told me it's just osmosis and there's basically nothing I can do about it. This made me sad as I've only had it resurfaced a few years ago. If indeed the vitamin C does work is it 100% an Iron issue and I should carry on trying to find someone (an expert!) who can come out and fix it for me!

I just want to also say that when I google black mark/fibreglass/osmosis that the marks do look fairly similar to mine, hence Im super confused.

thanks!!

 

[Texas Splash]

Vitamin C points towards iron. Are you on a well in your area? Old metal plumbing for your water supply perhaps? Do you notice any discolored/orange porcelain products in the house? It might also help to see a full set of water test results.