I recently changed over to RO/DI from tap water + prime and seems my skimmer is skimming less water and giving more of a dark green less watery liquid now...  Its the only thing I can think of that might have changed the performance of my skimmer, I have changed nothing else, could this be the reason for the change in performance?Â

Could be, but the main thing is you will be adding no contaminants (if the reading is 0 TDS) to your system like tap would. The main thing to getting a skimmer working is adjusting so you get a dry skimmate. This can be a challenge, so maybe you have met the challenge.