Quick question on a buubling effect.

My wife and I got a commission from a woman in our town who wanted a cauldron like I made in the craft section for her small haunt. Instead of a fog machine she wants an actual liquid that will bubble so I figured we could just run a bubble stone like used in a fish tank and a small pump to get the effect. However it doesn't give the thick slimy bubbles that she wants with just water. Any suggestions on what to use? I thought about corn syrup or mayber dish detergent dyed but that that might be a bit of a mess to clean up. Attached is a picture of the cauldron currently.

It still needs paint and a few touch ups before the liquid goes in.