Is the heat coming from the sun, the outdoor temperature, or the heat from being inside an attic?
If it's from the sun, you need to put a roof or something that keeps the sun off the tank.
If it's from the outdoor temperature, then you need to insulate the tank.
If it's from the heat of being inside an attic, then you need to increase the net free area of the attic/roof intake (usually soffits in the USA) and exhaust vents.
Depending on the outdoor temperature, you could install a coil of copper pipe that you run the water through to make the temperature of the water the same as the outdoor air. Set the coil up to run a fan through it, or directly on the least sunny side of your house like a giant radiator.
If the outdoor air temp is the problem, then using the ground is your best heatsink. Burying a lot of pipe a few feet underground and running your water through it would remove the heat.
Running a circulation pump through your DIY radiator on a timer might give you better results than trying to run all the water through it as you use it.
Best thing would be to just move your tank to the basement.