If your RO product water is pure, your aquarium should have minimal algae growth, a steady PH level, and low phosphate and nitrate levels. You can check the purity of your RO product water using a conductivity meter, TDS meter, or by measuring the general hardness and alkalinity of the water with a test kit. Purified water will have a general hardness of 0 GH and alkalinity of 0 dKH. If your test values vary from those described above, you may need to replace cartridges and/or the RO membrane.
It's not likely that you received a defective unit. But there are a few factors that effect the output of reverse osmosis units. These factors include; the source water pressure, temperature, and the amount of total dissolved solids within the source water.
The ideal operating pressure for a RO (reverse osmosis) unit is 65 psi (pounds per square inch). An operating pressure of 45 psi will reduce the output of a unit by 50%. There are pressure gauges available to monitor the operating pressure of the system. If the operating pressure is too low, an RO pressure boosting pump can be added to increase the operating pressure to the ideal range.
The temperature of the source water should ideally be in the 70° to 77°F range for optimal performance. A temperature of 50°F, or lower, will reduce the output of the unit by 50% or more. Either hot or a mix of hot/cold tap water should never be used for the source water due to the minerals that are added to the water from most water heaters.
Finally, the amount of total dissolved solids in the source water can reduce the amount of water produced by a RO unit. If your source water contains a higher concentration, it would be wise to install a mechanical pre-filter in line prior to the unit. These filters are available at most hardware stores. Because of the requirements of reverse osmosis units, it is wise to purchase a unit that is rated above the gallons per day that your needs require.