There is a res cooler which was very popular back in the O.G. days, and it was a sticky there for res cooling.
A hole the size of a small computer fan is cut in the roof of the reservoir at each end. A fan is placed face down blowing down onto the water on one hole: and shaded somehow, using creative diy means like the end of a small box that's larger than the hole, put over the hole and fan. Notches cut out of two sides so air can come in, and the body of the fan blocks reflection in from the side.
Same thing on the other end, shade the hole but let air out.
The fan is then just turned on.
The cause of this working is the 'evaporative effect'.
Heat in a column of water rises to the top: the moist air above the surface of the water is the place where the most heat collects: and humid air takes on and holds heat very well. The heat, rises up into the moist air, easier than water itself, evaporates: creating the 'evaporative effect' which is cooling which surpasses the actual loss of water, due to the convection of heat upward more easily, than the conversion of the water itself, into water vapor. It's the same principle that allows the swamp cooler or evaporative cooler, to chill buildings all over the world, from small cottages to warehouses of enormous size.
When you blow this hottest air off, a light evaporative effect takes place, that has been found to create minimal actual water evaporation yet it will take about ten degrees off a reservoir the size of a tub.
The sticky on Overgrow was many pages long; and everybody that did it said it worked. In my case i am able to use a real evaporative cooler to cool my water because i have access to the crawl space below my house and live in a very dry desert.
Others who have used the method i describe above will be able to verify what i'm telling you.