One of the CFL bucket scrubbers that I was testing began growing less and less algae, starting when it was about three months old. After five months, most growth had stopped. I did not think it could be the CFL bulbs because they looked fine, and they are supposed to last for years. But apparently this does not apply to algae growth, because after replacing the bulbs with new ones, growth immediately started again. These bulbs are cheap, so maybe a three-month replacement schedule should be followed.