I believe that warning is to replace the external tubing - you can get it on Amazon - there are two sizes:
Smaller size for the reagent line - 1.6mm ID (1/16")
Larger size for the saltwater line - 3.2mm ID (1/8")
Make sure to recalibrate after you replace them.
I have had mine running for 2yrs (two tests per day) and have not yet replaced the internal tubing - although maybe I should, 9 bucks actually isn't that bad.