It can be a lot of extra work but if you freeze water bottles and throw them in there it will help keep the rez temperature lower. Another thought is moving the bucket to a location in the shade if possible.

I'm not exactly sure what kind of system a dutch bucket system is, but make sure there are no openings in the reservoir (other than input/ ouput lines) to limit evaporation.
The other culprit is transpiration. If it's really hot and you have a lot of plants, they'll move a lot of water through them to cool off. As Ethno pionted out, cooling the water will help (cooler water cools more while moving less of it)but you may just have too small a reservoir for the number of plants being grown.