Found the answer on the Primochill website:
**DO NOT MIX: PC PURE includes an Anti-Microbial agent, we do not recommend mixing any other anti-algae or anti-microbial agents with the PC PURE fluid, this includes Liquid Utopia. Adding additional agents will cause a chemical reaction with the agent currently in the fluid. It is not dangerous, it will just cause the fluid not to operate at maximum levels and may cause unwanted build up.