What can I use to lower the density of water?

I've been modifying some of my cheap lava lamps, throwing away the old, cloudy master fluid and replacing it with distilled water. I realized that just water is too dense and the wax stays on top of a long while. So I've tried lowering the density by mixing water and alcohol, which works just fine until the alcohol evaporates and the water becomes dense again. Any other alternative that I can use to mix with the water?