In an aqueous solution, dissolved oxygen is the gaseous oxygen (O2) that is dissolved into it. Oxygen can get into water by aeration, as a waste product of photosythesis, and by diffusion from surrounding air. Low concentrations of disolved oxygen can stress aquatic life and can kill fish. Two easy ways to increase the dissolved oxygen in you pond or lake is by adding an aerator or fountain and or aquascaping with submersed vegetation.