paddytt wrote:
Thanks for all the replies. The water feature is a large (1.5m
diameter) sunken resevior, with a large 1.5m slate stone on top, with
water pumped up through the slate.

OK so no fish or wildlife at all are living in the water.

Green algae grows on the stone which is in full sun, and also on the
stone in the base, but the water runs clear. Scrubbing cleans it
(leaving some white and green marks), but the additives to keep the
algae away cost £5 a tub and on this size of feature, requires
treatment every 2 weeks in summer, once a month in winter.

A few water snails ought to do it for you in a more environmentally
friendly fashion and without the truly horrid smell of Jeyes fluid...

That the algae is growing so fast tends to suggest that for whatever
reason your "pond" water is full of nutrients. You might be able to make
some progress by eliminating nitrates and phosphates from the water.

Jeyes Fluid is good for cleaning algae off concrete and patio slabs, so
I wondered if I add a little to the water feature, will it keep it
clear of algae?

For a while - but can you stand the smell? Cheap hypochloride bleach is
another option to zap algae and probably smells less offensive.

Regards,
Martin Brown