By increasing the water's temperature we affect many other aspects of the ecosystem. Most importantly, the amount of oxygen dissolved in the water is decreased. Remember that fact for later. Also, the rates of photosynthesis and plant growth are increased. An increase in plant growth may seem to be a good thing at first.
More live plants means more dead plants later on.
The pile up of dead plants leads to an increase of bacteria, bacteria that consume oxygen along with dead plants. Therein lies the problem. There is now less oxygen and a greater demand for it. The warmer water also increases the metabolic rate of fish, which leads to a sharp decrease in the life expectancy of aquatic insects. The fish have an increasing the need for oxygen due to a rise in their metabolism and population.