Yes it's cyano ; one of the oldest form of life on Earth ; you can't eliminate it just keep it at bay ; I see you have a Juwel tank as I used to have and you have the same sand as I have used in my Juwel ; I think your sand layer is too thick , about the double of mine, which could explain why it has appeared so soon after starting the tank ; the sand have become anaerobic ; this kind of algae thrives in still water ; if you have the Juwel stock filter I can tell you that you don't have enough flow in your tank ; another thing is that your water is not oxygenated enough ; I don't know if you have enough plants ; and give them everything you got : pressurised CO2, liquid carbon and pump up the flow to get those nutrients to every possible leaf
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