Cleaning acrylic showers with Easy-Off Fume Free Oven Cleaner?

Nicotine stains come off with dollar store window cleaner with no scrubbing needed.

If it didn't come off with oven cleaner it isn't nicotine.

It's most likely minerals in the water. Most hard water stains look about the same color as nicotine stains do.

How easily hard water stains come off depends on how thick the stains are.(remember, you're trying to remove minerals and actual metal)

If nothing else works go get pool acid. (muriatic acid, AKA hydrochloric acid).

It's completely safe for fiberglass. You not so much.

Plenty of ventilation is not just a suggestion. It's really important.