Farmbird,
My guess is that you are not a professional cleaner, in which case the advice you have been given is not going to mean much to you.
For a 'kitchen cupboard' solution to the problem, first dilute some white vinegar (not wine vinegar), one part with three parts warm water. Use this solution to rinse out any detergents you may already have applied by spraying or dabbing onto the effected area, then blot well with a dry cotton cloth or paper towel.
Repeat this until you feel that the detergent has been removed, then leave a weighted absorbent pad over the area till dry.
When dry, if there is still visible staining, treat with a cloth dampened with methylated spirit and again blot well.
It a fishy smell remains after stain removal, lightly dampen the area and apply a liberal dressing of sodium bicarbonate, cover with a damp cloth and leave to completely dry.