What and When to Test
Which parameters and how frequently we test them often depends on how far along we are, and what we'll be keeping. We should always be aware of the inorganic nutrient levels in our tanks; but certain tests can wait until we add corals, and others might not be necessary at all. In this guide, I'll discuss the most commonly tested reef tank parameters, making some recommendations about when to test and how we might control their values.
In addition, how often we test certain parameters typically changes over time. Once you can accurately predict the values (prior to testing), you can usually check that parameter less often. But since reef tanks are constantly changing; it's still necessary to periodically monitor the most critical parameters, even when you think they haven't changed.
There are a variety of good kits and testing instruments available; and we often use different brands of test kits to check different parameters. For example, I use a temperature controller to monitor and control temperature, an API test kit to check ammonia, and Salifert to test: alkalinity, calcium, and magnesium. Over time, we usually develop our own personal preferences.
Like most parameters, temperature stability is very important. I usually recommended checking it when feeding your fish. We use heaters, chillers, and fans to control temperature, and thermometers to verify its value. A temperature controller can control the power to a heater, chiller, or fan when the temperature reaches a set value; plus they often have a digital thermometer and warning alarm.
In nature, corals are typically found in warmer waters (up to 86° F). Higher temperatures can increase the metabolism and growth rates of corals. However, since oxygen is more soluble at lower temperatures, we usually try to keep our tanks a little cooler (like between 78 and 80° F). Although, when temperatures are kept sable, it's generally considered safe to keep our reef tanks as warm as 84° F.
We use refractometers, hydrometers, and conductivity meters to measure salinity. For accuracy, I use RO/DI or distilled water to calibrate my refractometer to 1.000 sg. But I'll confess, I mostly use a swing arm hydrometer (which I checked against my calibrated refractometer).
Before each water change, I recommend checking the tank water's salinity, as well as the newly mixed saltwater (adjusting it as needed). And to maintain stability and help ensure that specific gravity (sg) doesn't exceed 1.027, we regularly replace the evaporated water with freshwater (typically RO/DI, or distilled). An automatic top off (ATO) unit can help automate this job.
Salt creep and other factors can also affect salinity, so occasionally spot check your tank's water, and correct it if necessary. While stability is important, most of our livestock is more tolerant to sudden decreases in salinity, than to sudden increases. To raise salinity, I usually recommend topping off the evaporated water with saltwater until the desired salinity is achieved.
We can use a test kit or a calibrated meter to monitor pH. But since we don't usually try to change its value, I rarely bother testing it. We're usually more concerned about keeping alkalinity stable than with achieving a particular pH. I usually only recommend checking pH when you are actively changing its value (like when dosing Kalkwasser, or using a calcium reactor).
However, excess CO2 lowers the water's pH. Therefore, low pH can indicate too many fish, poor aeration, or the tank being in a room with high concentrations of CO2. But assuming that the CO2 levels in the room are normal, a foam fractionator (protein skimmer) can increase gas exchange and help keep pH stable. Likewise, an attached macroalgae refugium, that's lit when your tank lights are off, can also help stabilize your tank's pH.
Notes: If dosing, you might choose to add alkalinity at night (when pH is naturally lower). Also, knowing your tank's pH level (in addition to the temperature and salinity) allows us to compute free ammonia from total ammonia, and vice versa.
Ammonia is a nutrient which can be utilized by algae (and reportedly, even corals) in our tanks. However, free ammonia (NH3) is toxic in higher concentrations, so we typically try to limit its presence. We're primarily concerned about the build up of toxic ammonia when setting up a new tank (including quarantine and hospital tanks).
But after the nitrogen cycle has become fully established, ammonia usually stays within a safe range, so there's typically little need to test for it. Still, there are events which can increase ammonia production. So we might test for ammonia if you see signs of distress or after a significant death, or even following a particularly disruptive event (such as a tank transfer, or disturbing a mature sand bed).
We usually measure total ammonia or free ammonia. The Seachem Ammonia Alert badge (shown above/right), continuously monitors free ammonia (NH3) for up to a year. Whereas a total ammonia test kit shows us the sum of free ammonia and ammonium (NH3 and NH4). The ratio between NH3 and NH4 is highly dependent on pH (with more free ammonia occurring at higher pH levels).
Notes: Here's a Total Ammonia Calculator (which can compute total ammonia from NH3), and a Free Ammonia Calculator (which can compute NH3 from total ammonia). See A Look at Ammonia for more information about ammonia in our tanks.
As part of the nitrogen cycle, ammonia is oxidized into nitrite. But since nitrite isn't very toxic in a marine aquarium, testing this parameter usually has limited value. Although, when your tank is in the process of establishing its nitrogen cycle, it can provide additional information about its progress. I'll propose that testing nitrite is mostly beneficial when tracking the stages of a fishless cycle (the dosing of ammonia or ammonium chloride to build up the biofilter on dry rock). But even then, this test isn't absolutely necessary.
Note: See A Guide to Reef Aquarium Cycling for more information about the nitrogen cycle.
Nitrifying bacteria oxidizes nitrite into nitrate; which is an important source of nitrogen for photosynthetic life. When keeping corals, you should always be aware of this nutrient's value, and try to keep it within the recommended range. I suggest testing for nitrate once a week, or at least until you can accurately predict its value beforehand. While certain inverts are more sensitive to high nitrate levels, fish are usually more tolerant; but try not to let it exceed 20 ppm.
Food is the primary source of phosphate in our reef tanks; but overfeeding can also add excess organics, which could result in problems. When keeping corals, adequate phosphate levels are arguably more critical than nitrate, and you should always be aware of its value. And like nitrate, you should attempt to keep phosphate within the recommended range.
Precisely determining phosphate levels (down to the hundreds of a part per million) can be challenging for any test kit. But the digital readout from a Hanna Checker makes it easy to read the results. Although, you can usually distinguish if phosphate is within the recommended range by using a traditional test kit (like from Salifert).
Low nutrient levels will end up starving photosynthetic life; so when keeping corals, higher phosphate levels are generally preferred over lower levels. And while higher levels can accelerate the growth rate of algae, it typically poses little other threat to our livestock. That said, phosphate levels shouldn't exceed 0.2 ppm.
Note: API's phosphate test kit is a high range test (which is better suited for a freshwater planted tank), and can't detect the lower levels that are within the recommended range for reef tanks.
If you are just keeping soft corals, you might not ever have to test alkalinity, as water changes (and dissolved rock and sand) will replenish most of the consumed elements. However, stony corals (LPS and SPS) and calcified algae (such as coralline) need alkalinity, calcium, and magnesium to form their calcified structures (making testing necessary).
At first, when the mass of stony coral is small, monthly testing of alkalinity might be adequate. And since alkalinity levels change more dramatically than either calcium or magnesium (and because its stability is more critical), you only have to test alkalinity to determine when you need to start dosing elements. But once you start dosing, you'll need to regularly test alkalinity, calcium, and magnesium (dosing them when necessary).
Note: It has been reported that when nutrient levels are too low, higher alkalinity levels can burn the tips of SPS corals.
Coral skeletons are made up of calcium carbonate, so calcium levels are important. Once you start dosing alkalinity, you'll need to test for and dose calcium as well. Like any parameter, it's best to keep this level stable. And while some salt brands mix calcium to over 500 ppm (which poses no risk to your livestock), I wouldn't feel the need to dose calcium back to more than 500 ppm.
Magnesium is consumed along with alkalinity and calcium. So after you start dosing alkalinity, you'll need to test for and dose magnesium as well. Also, like calcium, higher levels are usually well tolerated; but there typically isn't a need to dose magnesium past 1400 ppm.
Keeping the magnesium level within the recommended range allows our tanks to achieve the required alkalinity and calcium levels. Think of magnesium as a bucket which holds alkalinity and calcium; if the bucket is too small, it won''t be able hold enough of these other two elements.
Recommended Reef Tank Levels
|Temperature||76 to 83° F|
|Salinity||1.025 to 1.027 sg|
|pH||7.8 to 8.3|
|Free Ammonia (NH3)||Less than 0.02 ppm|
(NH3 + NH4)
Undetectable to 0.25 ppm
(at 8.2 pH or less)
|Nitrate (NO3)||3 to 10 ppm|
|Phosphate (PO4)||0.03 to 0.10 ppm|
|Alkalinity*||7 to 12 dKH|
|Calcium*||380 to 500 ppm|
|Magnesium*||1250 to 1400 ppm|
|*Ocean Values: Alk 7 dKH, Cal 420 ppm, Mag 1280 ppm|
Note: The above chart lists recommended ranges for coral reef tanks; certain values outside of these ranges might still be considered safe.
Dosing and Nutrient Export
Feeding adds inorganic nutrients to the water, which (along with other elements) are consumed by the photosynthetic life as it grows. Partial water changes replenish a portion of the consumed elements and export some of the nutrients. But in order to provide a healthy reef environment, we might need to adjust these parameters further (by exporting more of the excess nutrients, and by dosing insufficient nutrients and/or elements).
For nutrients, I recommend dosing them up to the minimum level of its recommended range, and exporting excess nutrients down to the maximum level of its recommended range. Additional ways to reduce excess nutrients include: binding agents, growing macroalgae, dosing carbon (like vinegar or vodka), or creating anaerobic zones to support denitrifying bacteria; but I won't be discussing these more advanced ways to export nutrients here.
As the mass of stony corals and coralline increases, partial water changes will no longer be able to replenish all of the consumed elements. These elements (as well as low nutrient levels) can be replenished by dosing them back into the water. For elements, I recommend trying to match the levels of a newly mixed batch of saltwater. This way, a large water change doesn't cause big parameter swings. If the levels of your salt mix aren't to your needs or liking, I'd consider switching brands.
For most reef tanks, I usually suggest sticking to dosing phosphate, nitrate, alkalinity, calcium, and magnesium (the parameters that we most commonly test for). People sometimes get into trouble when dosing other things. And only dose to replenish the elements which have been consumed (as reported by a test kit). You can start dosing alkalinity, calcium, and magnesium after alkalinity drops 1 dKH below its original value.
Notes: Dosing alkalinity and calcium together will cause a snowstorm of precipitation; so wait at least a half an hour before dosing the second element. Try to keep alkalinity swings to less than 1 dKH per day; dose multiple times a day if necessary (this can be automated with a dosing pump). Also, avoid increasing calcium by more than 50 ppm per day, or increasing magnesium by more than 100 ppm per day.
Through lots of experience and careful observation, it might be possible to delay (or even skip) certain tests. However, until we become experienced reef keepers, testing will always provide us with the best look into the inner workings of our reef tanks, and help us maintain stable water parameters (which is an essential part of reef keeping).