I have seen repeated threads over time where people just ran distilled water in an all copper loop and ended up with corrosion issues. I prefer to err on the side of caution and put 10% of pentosin into the loop with a few drops of non-alcohol iodine (I dare anything to try to grow!). I tried Zerex once, but it clouded the snot out of tubing in a matter of two weeks. Using pentosin, it take a few months to cloud up. I don't know what is in the Zerex, but I do know what Pentosin does not contain silicates. I also have found that Masterkleer stays a lot clearer than Clearflex or Tygon.