I had this same issue and tried many things, after a lot of research I found out most moss removal chemicals are toxic to wildlife/fish so I ended up settling on sodium hypochlorite (bleach) the good thing about this is that I was told it wont affect the wildlife due to the dilution apparently they use bleach for our drinking water too so I trusted that information. I got a brush from www.roofscraper.co.uk and used that to scrape the moss off I then used a normal broom and a bucket to apply the bleach solution. it has worked so far but after 7 months some algae has grown back so might have to find a more long term solution in the future