The northwest coastal area of Bali is currently for the development of marine fish hatcheries and cage farming. This study aims to influence the hatchery and aquaculture activities of marine fish cages in the northwest coastal area of Bali on seawater quality and plankton community. The parameters observed on water quality were total suspended solids, temperature, transparency, pH, dissolved oxygen, Salinity, ammonia, nitrite, nitrate, phosphate, C-organic, total organic matter, and plankton community on diversity, similarity, and dominancy index. The results of water quality observations found that water temperature was 27.5-29.1°C, TSS 0.028-0.046 ppm, transparency 7.4-12.8 m, pH 8.1-8.3, DO 5.4-6.9, Salinity 33.2-34.1, ammonia, 0.01-0.04 ppm, nitrite 0.028-0.041 ppm, nitrate 0.018-0.036 ppm phosphate 0.026-0.038 ppm, C-Organic 0.65-0.74 ppm, TOM 0.75-1.23 ppm. Found 38 genera and 10 phyla of plankton, with an abundance was 258-26,546 ind/L, an index of the diversity of 0.89-2.15, an index of similarity of 0.22-0.64, and an index of dominancy 0.18-0.43. From these results, it can be concluded that the coastal waters of northwest Bali are in the criteria that have not been polluted and are good for marine fish farming. The northwest coastal area of Bali must be managed properly so that marine fish farming can be sustainable.