A few months ago I installed 3 50 gallon rain barrels at my house - one set of two, and a singleton. We've been very happy with the results, but with the advent of hot humid weather we've gotten a fair bit of organic scum forming on top of the water when the barrels are full.
Are there additives we should be putting in the water to prevent this? Or some best practices that we should know about to keep this stuff from forming? We can scrape it off and hose down the lids every so often but would rather avoid it altogether.