Once a week will be tough. And I am going to assume that you would only consider a reservoir chilling system as a last-ditch effort.
Why don't we go through instead and try to discover WHY it is getting so hot!
You are right, that hot water holds little oxygen. First off, you will want to make certain that you have an airstone going in there at all times, especially while you are still working out your temperature troubles.
Now let's troubleshoot!

What is your AIR temperature?
How is your PUMP? Like, is the pump sized correctly for the amount of water it needs to move? Is it good quality, and in good condition? How is the pump filter? Is it cleaned weekly so as not to have to run harder?
Next, let's look at your RESERVOIR placement. Is the res getting direct light, or is it in the shade? Does it have a cover? Is there ANY way you can move it to the other side of a wall and put in an air conditioner, or can you drop the temps in your grow room low enough (68 night/78 day) that it cannot heat up that much?
Last, have you considered a conversion to a modified NFT system? In this type of system, you still use hydroton but instead of flooding it to deliver moisture, you install tubing from the pump that emits a constant STREAM of water over the hydroton, constantly mixing in oxygen, and allowing a LOT of surface area of the water to come into contact with the air. It will tend to stay close to the air temperature as a result, and you can take advantage of the evaporative action to strip heat from the water and media- as long as you have correct exhaust to remove excess humidity.