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.

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 =A35 a tub and on this size of feature, requires
treatment every 2 weeks in summer, once a month in winter.

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?

cheers
T