SUPPORT aquatic life. For maintenance of aquatic health, dissolved oxygen concentrations should approach saturation that concentration which is in equilibrium with the partial pressure of atmospheric oxygen. Dissolved oxygen can be measured in two ways: either in milligrams per liter (mg/L) or percent saturation (% sat). Water with an oxygen concentration of less than 3 mg/l will generally not support fish. When concentrations FALL to about 3-4 mg/L, fish start gasping for air at the surface or huddle around the water fall (higher concentration points). Bio-converter bacteria may start to die off dumping toxins into the water compounding the lack of oxygen to the fish. Levels between 3 and 5 mg/l can NORMALLY be tolerated for short periods. Young Koi are less tolerant of low oxygen than the older, larger ones. Above 5 mg/l, almost all aquatic ORGANISMS can survive indefinitely, provided other environmental parameters are within allowable limits. Colder water fish (such as trout) need levels above 6 mg/L, and may require levels above 7 mg/L for spawning. Warm water fish can usually tolerate levels as low as 4 mg/L. If oxygen levels dip down to 1-2 mg/L, even only for a few hours, it can result in large fish kills.