If your tank is suffering from high nitrate levels, the success of your reef will depend on your being able to get this under control. Changing 100% of the water would be the ideal, but it may shock your corals, fish and invertebrates in the process. A more gradual way is recommended.
Example from my 55-gallon Reef : Make up 20 gallons of fresh saltwater in a trashcan in front of your tank. Drain 10 gallons of tank water into the 20 gallons of new water, and let that mix. Pump 10 gallons of that water back into your tank, and let the power heads mix that water up in your tank for a minute or so. Then repeat this three more times. Dispose of the now polluted 20 gallons of water. Make up another 20 gallons of fresh saltwater, and repeat this procedure. As long as your temperature and salinity match the tank, your inhabitants won’t be affected adversely, and with each rotation of water, the nitrates are being diluted and removed from your tank.
Simply pulling out all of the water in one massive water change puts stress on your entire tank. Doing small water changes consistently won’t bring nitrate levels down. At best, it will maintain them at their current levels. Using the example above, a tank that was at 80ppm would be around 30ppm after a couple of hours work and your population will be happy and unaffected.
