Daphnia obtusa Kurz were cultured in 3 kinds of carbonate hard water, treated by micro-algae (Scendesnus obliquas Turp). The water fleas were harvested in two ways-interval and inoc-ulalting harvests. The results were shown as follows after comparison. In the presence of the algae severed as the pioneer organisms in the water, the water fleas could be inoculated. During the still culture stage without harvest, the population growth curves (N) show that N_c(in the improves fish-farming sewage) > N₀(in the pure fish-farming sewage) > N_i(in the improved ground water). During the stage of interval harvest, the mathematical functions of yield (the regression of the general number ranks accumulated from original data gathered each time) in the three kinds of culture medium were all shown to be Logistic growth curve and the population yields shown as N_c N_i N₀ and during the stage of noculation harvest: N_i N_c0 N₀.