A heating option is readily available. Open the covers . This is essential. You have to let the heat escape. If the heat accumulates on the surface of the water, then it will warm the moving surface even more.
Evaporation cools the water.
If you can increase the rate of evaporation you are actually cooling your water. Definitely the first step you must take. Doesn t need electricity. This removes the water vapours from the surface so more water will evaporate to reach the equilibrium point. Since your ventilators will keep on blowing the water vapors it is obvious that the evaporation will be kept to a maximum so the water will cool.