Can we assume you mean you want to make the fluid LESS dense?
I think making liquid less dense is dangerous thing to do as it involves flammable chemicals & not what lava lamp are doing.
What kero48 is referring to Ikana, is you don't make the wax more or less dense. You make the master fluid more dense (heavier) or less dense (lighter), this is what gets the wax to rise or fall. All of which can be done with not so flammable substances, ie, epsom salts, or PG. Yes, there are those formulations which call for "brake kleen" and more caustic and flammable ingredients, but the majority tend to want to keep the formulations and or tweaks simple and safe. Only thing that really can be done to the wax is to bleach it, color it. Master fluid surface tension can be decreased with SLES or SLS, which a little will cause the wax to stretch or tornado or column, you choose your reference here, or increases in surfactant will cause the wax to break into many smaller bubbles, less is more. Hope this helps, sure the other guys will chime in. Good luck!