Essential to successful reef keeping is a basic understanding of marine water chemistry and faithful monitoring of tank water chemical parameters. Take a breath, this is not quite as mentally challenging as it might seem. Here we can take you through it step by step...
Salinity is a measure of how much salt is in the water. The salinity of natural sea water varies by ocean, sea and reef, but is usually near 1.026 specific gravity or ~35 ppt salinity. The two most common instruments to measure salinity are the refractometer and the hydrometer.
Reef tank salinity should be maintained between 1.025 and 1.026 s.g. or 34 to 35 ppt salinity.
The concept of "pH" is complex and varies depending on the context in which it is discussed. Most simply put, it's a measure of the acidity (or basicity) of a solution. Solutions with a pH <7 can be called "acidic" with a pH>7 can be called "basic" with 7.0 being neutral. The pH of natural sea water varies throughout different marine environments but is generally 8.2 pH is one of the most important tests we can do to monitor the quality of our aquarium water.
Reef tank pH should be maintained between 8.0 and 8.6. pH in a reef tank will fluctuate throughout the day (rising during the day and falling at night). pH will be at its lowest point right before the aquarium lights come on and will rise throughout the day as dissolved oxygen levels rise in the tank and carbon dioxide is removed due to photosynthesis of algae.
Calcium is extremely important in reef aquariums. It is used by most organisms which we keep in our tanks to build their skeletons and grow.
Calcium should be maintained at 400 to 450 ppm
Reef tanks need calcium (not just for stony corals but for many soft corals and other reef critters too). Natural sea water has a calcium concentration of 400 to 420 ppm. There are many different ways to maintain calcium in reef aquariums. The 2 most common ways to maintain calcium in reef aquariums is through the slow addition of kalkwasswer (calcium hydroxide) mixed with RO water or through an ionically balanced 2 part calcium supplement like Two Little Fishies C-Balance. Two part calcium is the easiest way to add and maintain calcium to a reef aquarium and works just fine. Although it is more costly over the long run. Kalkwasser is a little more complicated to use but it is more cost effective and has many added benefits. Kalkwasser maintains both calcium and alkalinity and is ionically balanced. It also has a tendency to raise the pH so it will make your protein skimmer work better because protein skimmers are more efficient at higher pH levels. Kalkwasser will also help by precipitating phosphates. Phosphate in reef aquariums can cause hard corals to die, coralline algae to die and is food or a catalyst for nuisance hair algae and other undesireable algaes to grow. We basically do not want any phosphate in our tanks. Acceptable levels are 0.03 ppm or less with zero being ideal.
Buffers help protect your aquarium water against sudden changes in pH. The ability of the buffer to prevent sudden changes in pH is called a "buffering capacity." Alkalinity can be thought of as one measure of the buffering capacity of our aquarium water. Chemical compounds called "bicarbonate" and "carbonate" are the major contributors to alkalinity. They're not the only contributors, but in our tanks, they're by far the most important. They're also what are usually tested for in alkalinity test kits sold in the aquarium lobby.
Reef tank alkalinity should be maintained between 2.9 to 4 meq/L
It's important to note that while alkalinity can help your tank resist changes in pH, buffers can be "broken" if pH rises or falls too much too quickly. Therefore it is important to monitor pH so that it does not fall too far out of acceptable ranges.
Most corals sold in the aquarium industry are from the Caribbean or Indo-Pacific. These are warm, tropical reefs. Therefore, unless you know that your reef inhabitants are from colder waters, most reef aquariums should be kept at 75F to 84F.
As you will come to experience, pH, calcium and alkalinity interact with each other. Because of this, calcium and alkalinity must be measured at the same time and adjusted with respect to each other. Too much or too little of one is often a sign that the other is too low or too high respectively. Low pH can, though not always, be a sign of low alkalinity. It can also be a sign that the water is not being circulated adequately and CO2 is building up in the water. When your tank's water chemistry is not right in some respect, you may have to do some problem solving to figure out 1) what's causing the problem and 2) how to correct it. Generally water changes can help correct most water chemistry problems.
1) Go gradual: Don't try to correct an alkalinity level of 1 meq/L or a calcium level of 380 ppm overnight. Alkalinity should not be raised or lowered any faster than ~1meq/L per day and calcium adjusted no more quickly than +/-40 ppm per day. This is especially important since, remember, these two parameters interact with each other. Raising your alkalinity too fast could cause your calcium too plummet too quickly (and vice versa). Whenever you're adjusting alkalinity or calcium, be sure to monitor the other as well.
2) Test diligently: Always test the water before and after you act so that you know what you need to do next (if anything).
3) Observe: Pay attention to any patterns in your water chemistry. For example, before overreacting over high or low pH, take the time to measure your pH several times over the course of a day or two (test it in the morning, afternoon and night-- before the light comes on, while it's on and after it's off). It is normal for pH to swing in a range as wide as 8.0 to 8.6 over the course of a day (peeking at right before the lights turn off and falling at night). Of course if pH is excessively low (below 7.6) or excessively high (above 8.8) act immediately to correct it by the addition of buffer for low pH and water change for high pH.