What should I do if my grass carp have recently stopped eating? Please, experts, give me some advice.

Recently my grass carp haven't been eating much, they hardly touch the fish feed or grass feed, I haven't found any fish disease, maybe the water quality has deteriorated and turned dark green, using bleaching powder didn't have an obvious effect, could any friend give me some advice on how to adjust the water and what medicine would be better to use. Thanks a lot~~~!