The pond is 20X35 and 4-1/2 ft. deep. Gets shade all afternoon, have some water lillies, no other plants. No fish, just frogs that I want to keep. Have drained it 3 X's trying to keep it clean. It has three water falls and about 35-40 ft. of stream going to pond. Have tried barley, and some chemicals , but doesn't help. Afraid to use harsh chemicals due to dogs and frogs. Can you suggest anything to help please.