chipmunkofdoom2
Always Making Something
The prefilters and carbon blocks mostly protect the RO membrane from particulate matter and chlorine/chloramine. These should be replaced on a schedule every 6 months. If you want to replace them more intelligently, you can do so by replacing the prefilter when the pressure drops across the prefilters, and by replacing the carbon blocks when chlorine/chloramine shows up in the RO membrane waste water. This requires constant testing though. For most reefers, it's easier to replace the prefilters every 6 months.
The membrane can last a long time, depending upon your source water. Most membranes will last at least 3 years. With regular prefilter changes, my last membrane lasted more than 8 years. Only testing will tell you when it's time to replace an RO membrane. Test the TDS out of your tap, then test the TDS after your membrane (but BEFORE the DI resin). Divide post-RO TDS by the tap TDS. If the number is greater than 0.04 or 0.05, your membrane may need to be replaced.
The DI resin needs to be replaced whenever the TDS after DI resin reads more than 0 ppm. This is really difficult to quantify because it depends upon your tap water, the water's CO2 content, and how efficiently your membrane works.
So, long story short, you should expect to replace the prefilters twice a year ($40 in costs maybe) and the membrane every three years ($35 / 3 years = about $12 per year). The DI resin is hard to quantify. If you're in this for the long haul, a RO/DI filter will be worth it. The grocery stores by me all have very expensive distilled water, > $3.00 per gallon if I recall correctly. If you change 10g per month, that's $30 in water alone. 6 months worth of water changes will buy you a good RO/DI filter. Another 3 months worth of water changes will replace your filters for a year. So in 9 months or so, you will have broken even (not counting the cost of tap water).
