Try vitamin C before draining the pool.

It’s the cheapest test for metal stains—copper, iron, manganese. Organic stains (leaves, algae) usually fade with chlorine shock. If shock does nothing, you’ve got metal.

Crush a few vitamin C tablets, sprinkle them in a mesh bag or old sock, and rub it on the stain. If it lightens or disappears within a minute, you’re golden. For larger areas, buy ascorbic acid (pool stores sell it) and broadcast it across the water.

Don’t reach for the drain pump yet. Draining a pool is expensive, risks liner damage, and often exposes new stains. Vitamin C is a five-dollar fix that almost always works on metals.