Uv stetilizers irradiate free floating algae spores( green water), parasites, protozoa, and bacteria. Uv's irradiate them and in doing so eliminate their ability to reproduce. Some may be killed immediately but most are not. That's why it may take a while to work. For hair algae or film algaes try api algaefix. It wont kill whats there but if you remove what you can by hand then it will prevent it from coming back. If it's a slime algae or cyanobacteria try using erythromyacin to knock it out short term but always address the cause. You need to lower nitrates, phosphates, and silicates. Try keeping your light on only 4-6 hours a day as well. Many of the methods for eliminating slime 'algae' also reduce the prevalance of other types. If youre keeping delicate or scaleless fish its advisable to have a UV anyway.
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