If you are worried about the biodegradable part of the dishwashing liquid, you could use something like "Pink Solution". We can get that stuff here, as it is made on the West Coast (Vancouver). I use it on my hands and waxing implements. It is a kelp based natural degreasing cleaner. It cleans my applicators much better than any dishsoap. The only thing is that it smells like kelp. Perfectly harmless. As a matter of fact, you can eat it!!!
But I am not really worried about the small amount of dishsoap I am using, so I just use dish soap. After all, we're talking about 1 teaspoon once a year or something, so it's really no big deal.