Some systems will combine the first two stages into a single carbon block cartridge with a sediment filter wrapped around it. These cartridges work fine but don't last as long as 2 stage systems.

Third Stage - Reverse Osmosis
The membrane stage is where the bulk of the filtration happens. Water is introduced into the membrane housing placed under pressure. The membrane allows the passage of pure water while blocking any particles and compounds that are bigger than the water molecule of has an electrical charge the membrane will repel. This results two streams of output from the membrane filter housing, product (pure) water and waste water which contains all of the contaminants rejected by the membrane. The ratio of output from these two outputs when the unit is operating correctly should range from 1:4 (product : waste) to 1:6. In order to maintain the pressure of the output there will be a flow restrictor on the waste line.

Membranes are rated by 2 metrics, capacity and rejection rate. Capacity is measured in Gallons Per Day (gpd), so a 50gpd membrane will provide 50g of pure water in a 24 hour period. The % rejection rating tells you how effective the membrane will be. A good membrane will have a 96% rejection rate, this means that 4% of the contaminants in the water will not be removed. Membranes vary considerably in effectiveness from 90% in low cost units to 98% is select grade membranes, this can have significant effect on the quality of product water.

Membranes should last for many years, you can extend this life be periodically removing the flow restrictor from the waste line and flushing the membrane for 15 minutes periodically.

Fourth and Fifth Stages - Additional Filtration
Many RO systems incorporate additional post RO filters. De-ionization filters are a popular addition, the cartridge contains a mixed bed of anion and cation resins which use an ion exchange mechanism to remove impurities that pass through the RO membrane. Examples of these impurities include silica, phosphates and nitrates as well as dissolved gasses like ammonia and carbon dioxide. Special types of De-ionization beads are available that change colour as they are exhausted and others that are optimized to combat specific contaminants like silica.

The useful life of a DI cartridge is primarily determined by the efficiency of the RO membrane. The lower the contaminant level in the RO water the less work the DI filter has to do. Other factors also impact the useful lifespan of DI cartridges, the primary one being dissolved gas levels in the water. In cases where dissolved CO2 is high, this CO2 will be removed by the DI cartridge. Additionally water supplies that use chloramines will have an effect . The carbon block in your pre filter will destroy the chloramines and remove the chlorine but the ammonia that was part of the chloramines will need to be removed by the DI cartridge.

There are also granular carbon filters that are used post RO membrane in systems that are designed for drinking water. This filter is used to polish the taste of the water and is not required for aquarium use.

Optional Components - Helpful Addons
There are some things that will help you get the most out of your RO or RO/DI system.

Total Dissolved Solids Meter
A TDS meter is really a must have addition to your filtration system. The meter measures the amount of charged ions in the water in the same way a conductivity meter does. It converts this measurement to a measurement in units of parts per million based on one of several standards. You can use a standard conductivity meter in place of a TDS meter.

TDS meters should be used to measure the output of your RO membrane in order to confirm your membrane is working correctly. For example if your source water has a TDS reading of 200ppm and your membrane has a rejection rate of 96% your RO product water should have a TDS reading of 4ppm. A reading much higher of 4ppm would be an indicator that your membrane is not meeting its potential.

You should also use your TDS meter to check the output of your post membrane DI cartridge. When the TDS measurement starts to climb it is time to change your DI cartridge.

Pressure Gauge
A pressure gauge added to your system after pre filters and before the membrane will let you confirm that you have sufficient pressure in your system for the membrane to work efficiently. Should your pre filters become clogged the pressure will drop and you will know it is time to change them.

Chlorine Test
A chlorine test kit is required to make sure your carbon filter is protecting your RO membrane. Any chlorine detected at or above 0.1ppm mandates the replacement of the carbon block.

Automatic Shut Off
Using an RO filter to automatically fill a vessel of some kind is very valuable. You can use it to keep extra water on hand in case it is necessary to do an unplanned water change or fill a vessel used for top up water to replace evaporation in the system. It is not advisable to plumb a RO unit directly to a system for top ups in case the shut off system fails. You could have a flood and dramatically lower the salinity in the system, harming your livestock.

Some systems will simply connect the output of the RO system to a float valve in the target vessel. This is prone to failure because the system is still pressurized when the valve closes. Eventually the float or some of the tubing on the RO unit will fail causing a flood. An Automatic Shut Off valve is added to the product line but also attaches to the feed line so when the float closes the pressure on the output will trip a switch that also shuts down the feed water to the unit. You can also add an electronic float switch to the target vessel that will activate a solenoid to shut down the feed water. This is usually more reliable than a mechanical float but will require a bit of Do It Yourself work.

Water Alarm or Leak Detector
A water alarm is pretty useful when you don't have an automatic shut off or would like some extra protection from floods. The water alarm has two contacts that sit at the top of the vessel you are filling or on the floor next to it. When water touches the contacts it completes a circuit and sounds an audible alarm. You can also take this one step further adding an automatic shut off solenoid to the system that cuts feed water to the unit when water closes the contacts.

Getting the Most Out of Your System
RO membranes are designed to work in specific and relatively narrow conditions.

Water Pressure
Most RO membranes will work in a range between 40 psi and 80 psi with best results at 60 psi. Your pressure gauge should be installed between the pre filters and the membrane. If the pressure drops below 60 psi replace your pre filters and test again. If your pressure stays low you may have to purchase a booster pump to additional water pressure. A good booster pump will come with a pressure switch that cycles on and off the pump as needed to provide sufficient pressure. If your pressure is too high you may have to invest in a pressure regulator valve.

Temperature
Membrane effectiveness varies with temperature. As temperature decreases the viscosity of the water increases, reducing output. Target temperature should be 77F with a maximum of 100F to avoid membrane damage. In the case of permanent system installations it is common to plumb the system into the cold water supply. This does work but often causes a decrease in output especially in the winter. It is not unusual to water temperatures of 55F on the cold water line in Edmonton and this can reduce output by as much as 35%.

Water Softeners
Water softeners work though ion exchange much like a DI filter does. It is best to not use softened water as a source for the RO system because the softened water contains many salts that will increase membrane fouling and DI filter exhaustion.