Well, after all this time finally a happy ending.
After about 18 months of trying to get rid of algal growths in my fibreglass keel water tank, I have finally found the solution. It is a product (sold here in Oz) called Acquasafe, made by Davey (the pump people). It contains hydrogen peroxide and trace amounts of silver ions, and is designed for treating potable water in rainwater tanks
, so is completely safe to ingest. No doubt there are equivalent products in other countries.
What a relief! I shock-dosed it with bleach a dozen or more times to no avail - after a week or so the water would be gooey again with algae and be undrinkable.
Still got that hint of fibreglass, but it's fiine for coffee or cooking
and still ok to drink au naturel in a pinch.
Oh and $40 worth will treat 150 fills of my tank, so not exactly expensive!