This usually means iron bacteria, which live on the iron in the water rather than being a mineral themselves.

The bacteria feed on dissolved iron and leave a gelatinous orange mass behind, and the oily rainbow on the surface is the film they make, not fuel.

Look for orange jelly on the toilet tank walls and flapper, a rainbow sheen that breaks into pieces when poked, and clogging that comes back after cleaning.

Do not buy a softener for this. Exchanging hardness minerals does nothing to a living colony, and the new resin bed will foul the same way the tank did.