Recently, something has shifted causing gravel to get green. Assuming algea. Tank is mature, no hair algea, some sprigs of bryopsis. Rarely do water changes but have done 20% in 2x in last month (100 gal tank), went black for 3 days, turned off t-5s and cut led to 30%. My guess is a chemistry issue or green is cyano not algea, not matted or hairy. Di cartridge is showing 0 on meter, but could be some micro element. Not sure on what to try next. Dropped feeding too 2x a week, low fish load