Water Quality for Reef Aquariums

A reef aquarium filled with many different species and types of coral can be a very rewarding and enjoyable experience. Water quality is the most important aspect of keeping a thriving reef environment that is both suitable for the fish and invertebrates while keeping the tank visually pleasing The primary aspects of water quality do not differ much from keeping fish-only aquariums. However, the addition of corals will add a new dimension with more parameters to monitor for healthy, growing, and vibrant corals and fish.  These include calcium, alkalinity, magnesium, phosphate, pH, ammonia, salinity, and temperature.


An often overlooked aspect of successful reef aquariums is your water source. One might think that water from the tap is just fine because of most municipalities have strict water quality requirements. While tap water is typically safe for drinking it is not adequate to support growth of coral or fish. It often contains chloramines, chlorine, nitrates, phosphates, and dissolved metals that are hazardous to marine invertebrates.  The addition of water conditioners alone do not make it safe for these sensitive  creatures and can lead to nuisance algae overgrowth issues. The solution most reef keepers practice is to use reverse osmosis deionized (RODI) water. RODI is water that has been treated by a reverse osmosis filter and deionization resin that removes impurities and results in pure freshwater. This pure water should be the source of your top-off (water used to replace what evaporates) and water change regime. Keep in mind that RODI filters create freshwater, and the addition of synthetic reef salt at the appropriate levels is required before adding to your system when water changes are performed. Most aquarium specialty shops sell both pure RODI freshwater and premixed saltwater created from RODI freshwater. Alternatively, some hobbyists purchase their own RODI water systems for home use.


Many corals and saltwater invertebrates use the dissolved calcium in the water to grow and form their skeletons. As the corals grow this calcium will get depleted and will need to be replaced. There are many different commercially available test kits that can be used to monitor calcium levels. In new reef aquariums it is advised to test these levels weekly to keep levels within range. The average level of seawater calcium levels is approximately 420 ppm. Acceptable levels in reef aquaria lie between 380-450ppm. Boosting calcium levels beyond this level does not increase coral growth but keeping it low will hinder growth. For some reef aquariums, a regular schedule of water changes with a high quality synthetic reef salt is more than enough to keep calcium at the appropriate levels without additional additives. Other options include direct supplementation of calcium either with commercial products or by adding a calcium reactor or a Kalkwasser reactor to the system.


Alkalinity is a measure of how many hydrogen atoms (H+) is needed to lower pH to around 4.5. This is a surrogate measurement of bicarbonate concentration within the water. Bicarbonate is also needed for most corals to build their skeleton and grow. Like calcium, bicarbonate gets depleted as it is used from the water column. The suggested range for alkalinity is between 2.5-4meq/L (7-11 dKH, 125-200 ppm CaCO3 equivalents). While low levels of alkalinity will depress coral growth, high levels can actually decrease calcium concentrations and cause precipitation of calcium carbonate out of solution and onto aquarium equipment such as pumps causing poor performance and a need for increased maintenance. Alkalinity also works to buffer the aquarium to help keep the pH stable. Alkalinity should be tested as frequently as calcium. Balanced calcium and alkalinity additives are available that will help keep these values within the correct ranges when used appropriately. As with calcium levels, a regular schedule of water changes with a high quality synthetic reef salt is more than enough to keep alkalinity at the appropriate levels without additional additives.


Corals and marine fish live in saltwater. Salinity is the measure of how much salt and other minerals are dissolved within the water. This can be measured in various ways but the most common is by a hydrometer or refractometer and measured in parts per thousand (ppt) or specific gravity. Natural seawater has a salinity of about 35 ppt and a specific gravity of 1.026. Within reef aquariums it is best to keep levels stable between 34-36 ppt or a specific gravity of 1.024-1.027. Keep in mind that these measurable levels can change slightly with changes in temperature. It is best to keep your aquarium at a consistent temperature to ensure testing is consistent. Large swings in salinity can be detrimental to both fish and coral health. It is advised to premix saltwater with RODI before performing water changes. Many reef keepers will utilize automatic top-off systems to prevent the salinity from changing too much with evaporation as well.


Temperature is important within a reef aquarium for a number of reasons. Many fish and corals are adapted to a specific range and will not do well outside these ranges. Less oxygen is available for aquarium inhabitants at higher temperatures. Coral and fish metabolism also increases with increasing temperature meaning they require more oxygen. These two concepts compound each other and can lead to low oxygen levels in a high temperature or overstocked tank attributing to coral and fish decline. The recommended temperature for a reef aquarium is between 76-80° Fahrenheit (24-27° Celsius). The more stable the temperature the better. Aquariums heaters have internal thermostats that keep aquariums at a consistent temperature. Aquarium chillers are also available if excessive heat is a problem. Monitoring temperature with a reliable aquarium thermometer is recommended as heaters and chillers can fail.


Calcium, alkalinity, and pH are all closely related and in a constant balance with each other all affecting the calcification of corals. Low pH can make the calcification process more difficult for corals while a high pH can cause dissolved calcium to drop as it starts to precipitate out of the water.
Seawater has an average pH of around 8.3., so our overall goal is to try to achieve as close to this as possible within a pH range of 7.8-8.4. Any changes in pH should be made slowly and it’s important to strive for constant pH.
One of the many factors that can affect pH in a reef system is light.
Big factor that can influence pH is the amount of carbon dioxide dissolved in the water. Just like land animals, aquatic animals “breathe” out carbon dioxide. If carbon dioxide starts to build up in aquarium water it can cause the pH to drop. The best way to keep carbon dioxide levels low is to provide plenty of aeration to help the carbon dioxide escape the water.
One thing you may notice still is the pH changes from day to night in many reef aquariums. During the day plants and algae in the aquarium produce oxygen and absorb carbon dioxide as they undergo photosynthesis. At night photosynthesis stops and both plants and animals in the system just produce carbon dioxide. What this means is during the day the pH will go up and overnight it will go down. Aeration alone doesn’t often help prevent this pH swing. Making sure you have good alkalinity levels to buffer the pH is important. Many hobbyists will further try to eliminate this daily swing by having a smaller tank, called a refugium, attached to the main tank filled with macroalgae with a light cycle opposite of the main tank.


Magnesium is important in reef aquariums due to its interactions with calcium and alkalinity. Magnesium acts to counteract the precipitation of calcium carbonate out of seawater. This keeps the water saturated with calcium and keeps it available for corals to grow their skeletons. Seawater concentrations are around 1285ppm. Within reef aquaria acceptable ranges are from 1200-1400ppm. Additives are available to supplement if needed. If large adjustments need to be performed, a gradual change to the correct amount is recommended. For many reef aquariums, a regular schedule of water changes with a high quality synthetic reef salt is more than enough to keep magnesium at the appropriate levels without needing additional additives.


Phosphate is something that should be kept low in reef aquaria. At higher levels it will inhibit coral skeleton growth. It also is needed by algae to grow so if kept at low levels it can help keep nuisance algae growth to a minimum. Phosphate should be kept at levels below 0.03ppm. The best ways to keep phosphate low include growing and harvesting macroalgae from your tank, using an efficient protein skimmer, using phosphate binding media, appropriate stocking levels of fish, and feeding foods low in phosphate. One or all of these methods can be utilized to keep phosphate at appropriate levels.


While ammonia is part of the basic nitrogen cycle, it is important to be mentioned again here. Even at very low levels, ammonia is harmful to fish and corals. Under most mature aquarium conditions ammonia is detoxified rapidly and is of little concern. It can become a problem when adding new livestock such as fish, corals, or live rock, but also if a fish or coral was to die within your tank and begins to decompose rapidly. These can all cause spikes in ammonia levels. Successful reefers believe acceptable levels of ammonia are undetectable levels of ammonia. This is generally considered to be less than 0.1ppm. Ammonia levels as low as 0.2 ppm can be dangerous to fish. Keeping a keen eye on ammonia is important. A healthy biological filter will work to help keep ammonia levels within the optimal range.


As keeping corals in aquariums keeps evolving and more knowledge is acquired, the detection and manipulation of different elements is becoming more commonplace. Iodine, iron, strontium, and boron are a few of the more common elements tested for. The novice reefer does not need to test for and alter these element concentrations beyond the levels present within quality synthetic sea salt mixes. As stated previously, for many reef aquariums, a regular schedule of water changes with a high quality synthetic reef salt is more than enough to keep trace elements at the appropriate levels without needing additional additives.


The most important aspect of reef keeping is water quality. If water quality is not appropriate and consistent corals will not be vibrant and grow. Recommended testing intervals can vary. However, regular monitoring of water quality parameters is vital. Weekly testing for most parameters is sufficient. A log of test results should be made to provide reference to trends over time and with any changes or additives made to the tank. Daily water testing is not unreasonable and is encouraged to promote the best habitat possible when trying to supplement your tank beyond what a quality synthetic salt mix provides. Start simple and work towards advanced techniques and equipment. Patience is a virtue in this hobby, doing too much too quickly will often result in disaster. Many do not consider a reef aquarium mature until 6 months to 1 year after setup. The wait is hard, and progress can be slow, but a healthy, mature, growing reef tank is awe inspiring and enjoyable for years to come.