based on what I have learned, cyano forms as a result of stale/stagnant water flow. meaning the places it forms has too low of flow/not enough oxygen. happens for me in tall tanks mostly. there is a relationship between phosphates and cyano but the debates on that theory make me wonder. you can treat easiest with ethromycin and it will be gone, but if you don't figure out what the cause is (which for me was low flow) it will come back. Once I added an airstone, the cyano disappeared within about a month. I did it this way instead of adding em because I don't like using chemicals/medicines in my tanks.