R/O Water Test???

[Aquattro]

Ideally you should test your TDS every month, once you get above 1 or 2ppm output, I'd change membranes. Here in victoria, our water is really soft, so I can get away with a new membrane every 18 months. My carbon and prefilter get changed after my pressure through the unit drops 10psi. If you got your's used, or you're unsure, get a new carbon block and prefilter at the minimum.