This study was conducted at the Fish Laboratory,Animal Production Department, College of Agriculture, University of Sulaimani using commercial dry yeast at four levels (zero, 3, 5 and 7%) to study their effects on the microbial load in fish aquaria water,total weight gain (g), daily growth rate(g /day) and relative growth rate(%)as well as the pH of aquaria water.The experiment included four treatments, each in three replicates (plastic aquaria) in which 10 finger lings of common carp of the same weight (3.5 gram) were stocked in each aquarium. The actual duration of experimental feeding trial was three months. The results showed that 7% of dietary yeast gave the best result of total weight gain, daily growth rate, and relative growth rate. Moreover, it had the lowest microbial load which decreased from 6.2*10 5 to 3.15*10 4 at the end of the experiment as compared to the control without dry yeast that increased from 2.01*10 3 to 5.4*10 3 .No significant differences were observed among the treatments in total weight gain (g), daily weight gain ( g/day) and relative growth rate (%). The pH value ranged from 6.8 in the control treatment to 7.4 in the second treatment (5% yeast). Accordingly, dry yeast could be used safely in feeding common carp up to 7% to attain the best microbial load.