A Brief Primer on Water Chemistry

Measuring the parameters of your aquarium can be a bit daunting if you don't know much about chemistry. However, a basic understanding of what's happening with the water in your tank can be instrumental in preventing things from going wrong (and knowing what action to take when it does).

Keep the Water Constant
Saltwater fish (and coral) are much more sensitive to fluctuations in the water than freshwater fish. Freshwater fish live in an environment that is very dynamic. If you live in a little pond, you have to adapt to varying amounts of rainfall, evaporation, and large temperature swings. If you are a fish living in a coral reef, you don't have to adapt to any of these things. The ocean is an enormous body of water. The immense size of the ocean acts as a giant buffer; it takes a lot more than a thunderstorm or a cold winter to change the water in the ocean. Therefore, if you only learn one thing about the water in your reef aquarium, remember that your fish and coral HATE change. The absolute values of the different parameters of the water (e.g. specific gravity, temp, pH, etc.) don't matter as much as preventing them from changing.

Testing Your Water and Test Kits
Everyone has different opinions on how often to test their water. I test a bit more (about once a week) but this is because our tank is still young and we're still learning the ropes. As a rule of thumb, test your water before and after each water change to keep track of changes. And check the temperature on a daily basis, especially during the heat of the summer, when temps can soar (more on this in a later post).
Important parameters to check include:

• Temperature: 78-80 degrees Farenheit
• Specific gravity (or salinity): 1.024-1.026
• pH: 8.1-8.3
• Nitrates: as low as possible (0-10 ppm)
• Ammonia and Nitrites: 0 ppm
• Phosphate: as low as possible (below 0.05 ppm)
• Calcium (reef tanks only): 375-475 ppm
• Alkalinity (reef tanks only): 2.5-3.5 meq/L or 7-10 dkH
  

Water Parameters for our Reef Aquarium:

• Temperature:
• I've seen anything from 75 to 83 (Farenheit). The important thing is to keep it stable (+/- 1 degree). Ours runs at 80 (now that we have a fan in the hood to help cool the lights). I'll allow it to go 79 in the winter but that's it. Keep in mind cooler temps prevent algae growth. The actual temp depends on the species you keep in your tank. It seems reef tanks run a bit higher (around 80 vs 78) than fish-only (or FOWLR). It's easy to maintain with a heater and a thermometer. The hard part is cooling the water off. Temps can rise, especially with lights (and especially in the summer). A strategically placed fan in the hood and/or sump usually does the trick (our hood fan lowered the temp by 2 degrees). The downside? Our fan is cooling via evaporation so we have to refill the tank more frequently (about every other day). Also, temperature is the one thing I recommend checking daily (I actually check it 2x--am and pm).
  


• Salinity:
• This is measured by specific gravity. Recommended values are 1.023-1.026 (low end recommended for fish-only systems, higher end recommended for coral). Interestingly, a lower specific gravity retards parasitic growth. This is why aquarium shops often keep their fish housed at a specific gravity of 1.023 (or lower), not to mention why it's so important to carefully acclimate your new fish before dumping them into your tank! Also, keep in mind that while water evaporates, salt does not so when the water levels in your tank lowers, the salinity actually increases! That's why topping off with freshwater is critical. Also, smaller, more frequent top offs (daily) prevent large fluctuations in salinity. I use a refractometer to measure the specific gravity. Remember that temperature affects specific gravity so you may want to invest in one that is set to your tank's temp to avoid extra math in converting values. What exactly is specific gravity and what does it have to do with the percent of salt in the water? Specific gravity (or relative density) compares the density of the saltwater to pure water (therefore, pure water has a specific gravity of 1). Because saltwater has a lot of minerals in it, the specific gravity is higher than that of freshwater. We keep our tank at 1.025. I measure it with our refractometer about every other day. Convert your specific gravity to ppm here
  
  pH:
• This is the measure of hydrogen ions (H+) in the water. Freshwater is neutral (pH=7). Saltwater is slightly basic (our tank has a pH of 8.3). Again, keep your pH constant and your fish will thank you. pH is especially important for the calcification process, important for corals (especially stony ones). Interestingly, the act of photosynthesis conducted by your coral and algae in the tank will increase the pH. Therefore, expect small, normal,daily fluctuations when the lights go on (pH will rise) and off (pH will drop). This fluctuation should be no more than about 0.2. Briefly, because photosynthesis consumes CO2 and releases O2, this results in a pH increase (more basic). Once photosynthesis halts (when the lights go out), this process reverses, resulting in a slight acidification. I recommend using a pH probe to more easily test the water rather than the cumbersome test provided in many kits. More on pH here.
  
• Ammonia, Nitrites, Nitrates:
• This is basically produced by fish waste and, unfortunately, is very toxic to your aquarium denizens. Luckily, all the live rock you put in your system acts as a powerful biological filter, utilizing beneficial nitrifying bacteria to convert the ammonia (NH3), then nitrite (NO2-) and finally, the less toxic form, nitrate (NO3-). (More on this nitrogen cycle later). Nitrates are less toxic than ammonia but promote algae growth and are harmful at higher levels. Ammonia and nitrite levels should always be 0. Nitrates can be tolerated at slightly higher levels (~10 ppm). You can significantly lower your nitrate levels with excellent filtration equipment and live rock as well as small, frequent water changes. Also, NEVER overfeed your fish. Scavengers, such as hermit crabs, snails, and shrimp, can also aid in getting rid of excess debris before they produce these harmful chemicals. I test the nitrates before and after a water change and the ammonia and nitrites only if there's a problem.


• Phosphates:
• Ideally, this should be as low as possible (below 0.03) because, quite simply, phosphates promote algae growth. Phosphates (PO4) are typically introduced in the fish food. Macroalgae, frequent water changes, and feeding your fish the correct amount will prevent phosphate spikes.

• Calcium:
• This element is important in a reef aquarium simply because your coral (especially stony ones) utilize it to build their skeletons. Hey, just like us! Ideally, you want 375-475 ppm. Calcium chloride can be added to the water to boost levels but be sure to be extra conservative when adding anything to your water. Add only 1/4 the manufacturer's recommended dose and test levels every 24 hours. Adjust every other day until desired level is obtained. Watch your aquarium carefully 24 hours after adding anything new to the water for signs of distress or change.


Alkalinity:

• This measurement is often confused with pH. Alkalnity is the ability of your aquarium to buffer changes in pH. In seawater, this is controlled by bicarbonate. Therefore, calcium and alkalinity are closely related when it comes to your tank since corals use bicarbonate for the calcification process required to build their skeletons. For reef aquariums, 2.5-4 meq/L is recommended. To find out more about alkalinity, go to this link: http://en.wikipedia.org/wiki/Alkalinity.

More links on water parameters for reef aquariums:

http://www.reefkeeping.com/issues/2004-05/rhf/index.php
http://www.thereeftank.com/articles/understanding-reef-tank-water-testing-and-parameters/http://reefchronicle.com/chemical-parameters-for-a-reef/
http://www.melevsreef.com/parms.html