Vitamin C made stain worse?

[quadricorn]

So I had a yellowish stain that I originally thought was just algae but it wouldn't brush off. A quick search said it could somehow be iron and to try some ascorbic acid. I chucked some vitamin c tablets on it, but the tablets either stained the pool or made things worse.

Any ideas on what the original stain was or how I can fix the worsened stain.

 

[Texas Splash]

Copper maybe? Do you use a cartridge filter? If so, what color does it look like? Do you use any copper-based products? Maybe try applying some dry acid to a test spot if you have some available to try.