I believe he meant to say add salt to the water bottle as this lowers the freezing temp...good idea if you want to do bottles but bottles suck. Too much work, there is a couple good ways to insulate your reservoir but thats a great help.

Do a search on ice probes....its an aquarium chiller, thats what I used when I did DWC. It will set you back about 120 to the door and will drop temps in an 18 gallon 10 degrees below ambient temperatures.

Hope this solves your delima.