Low flow, phosphates,Nitrates, DOC
, wrong spectrium lighting(old bulbs) are all factors in cyno. Start with one issue and go from there. If you feed a lot cut back if that does not help maybe add another power head if that does not help maybe some new bulbs and so on till its gone. Work on lowering the nitrates and when doing a water change suck up the cyno with a hose or turkey baster. Try not to blow it around the tank that will only help it sperad.
Its a battle you want to get started on sooner then later.