This research investigates how using algae as an ecotoxicological test species is easier than using daphnia for identifying toxic causative substances. From the results of the ecotoxicity measurements on the Shihwa lake and its tributaries, heavy metals were considered as one of major factors in causing toxicity. The algae ecotoxicity value was 9.6 while the daphnia ecotoxicity value was 0.8 in the Jeongwang stream. By using algae as the test species, we could identify the toxicity that causes heavy metals which might otherwise have been missed with only daphnia. The results from the EDTA addition test showed that zinc and copper were the main toxic causative substances in the Jeongwang stream and Gunja stream.