There used to be an industrial biocide, Tego51, that was preferred by Catteries, as it was cat-compatible.
As far as I know the company that sold Tego51 has been sold at least twice since I knew them. Maybe check the Diversey UK website to see if it is still available or has been superceded.
Hypochlorite would prevent algae forming I suspect but it isn’t good for the feline.
Are there any cat friendly alternatives?