How bigs your tank pwc lighting and feeding are all good places to start..But the causes is high nutrients phosphate, nitrate, and they also consume heavy metals .depending on the size of your tank API makes a phosphate absorbing material in a bag for tanks up to 55 g and its realy cheap...if u have a reactor run gfo.... U will need to run some carbon to remove any metals if some our present ...while cutting your lighting will slow the spread ,and less feeding will lower nutrients,pwc help to but it will take 5 days of consecutive 15% pwc at a min to make a difference best bet is to absorb the excess nutrients with media...