When I came back to reefkeeping in 2006 the desired range of some of these parameters had changed and there were other parameters that were equally as important. I had worked for several years in a laboratory and understood chemistry but I still found it a little confusing! This article will tell you what you need to know and give you the resources to learn more about the chemistry if you desire.

The first thing that many people wonder is what salt should I use? I will propose that there is an even more basic question to ask yourself. “What water shall I use?“ For the vast majority of us the answer should be reverse osmosis/deionized (RO/DI) water. City water contains chlorine or chloramine that is used to make it safe for drinking; both city water and well water contain many other elements and compounds such as copper, nitrate and phosphate that we don't want in our aquariums. RO/DI water from a reliable source is free of these impurities. Many LFS's (local fish stores) sell RO/DI water but often hobbyists buy their own unit. Whether you are purchasing water or making it you should have a TDS (total dissolved solids) meter to ensure that the water you are using is pure. You will use this water to replace the water that is lost due to evaporation and to make new saltwater.

Now that you have your water which salt is best? To me, salt brands are like ketchup brands, people will argue about the relative merits of one over another but you can dip your french fries in any of them. People have successful reef tanks using many different salt brands. I've used several brands and have never noticed any major difference in the health of my livestock. Reef salts have higher calcium levels but are more expensive that non-reef salts. Personally, I buy a non-reef salt and dose calcium. I have heard many people advise against purchasing no name salts made in China as they may have impurities at levels that are hazardous to your fish. The articles that I have included in the resources section contain additional links if you are interested in more in depth information about salts.

The average salinity of natural sea water (NSW) is 35 ppt or a specific gravity (SpG) of 1.026.  Regardless of which brand of salt you use you should keep your reef tank between 1.024 – 1.026. In years past it was believed that a lower salinity was less stressful to fish but most experts now recommend keeping even fish only tanks at around the salinity of NSW.

The best, most accurate, way to measure salinity is to use a refractometer with automatic temperature compensation. Floating hydrometers are also accurate but can be more difficult to read. A couple of recent articles by respected authors have disputed the conventional wisdom that swing arm hydrometers are inaccurate.

I agree that these inexpensive hydrometers can be accurate but in order to remain accurate they must be properly read and maintained. This means that you must follow the manufacturer's directions exactly, set it on a flat, level surface to read,  and make sure every time you use it that there are no bubbles clinging to the swing arm. To remain accurate the hydrometer must be rinsed with RO/DI water each and every time you use it. Because of this many people choose to use a refractometer, especially since they are now so reasonably priced.

Temperature is another important parameter in your aquarium. Some fish have special needs but a mixed reef aquarium will do well with a temperature between 76 – 82 degrees Fahrenheit (24.5 – 27.5 C). Although your tank's temperature may fluctuate a degree or two when your lights come on within this range a stable temperature is as important as the exact temperature.

You will need to test for ammonia (NH3), nitrite (NO2) and nitrate (NO3) which are all part of the nitrogen cycle. Cycling will be covered in another article. For the purposes of this article what you need to know is that the desirable level of all three compounds is zero. A nitrate level of <10 ppm is acceptable for a fish only tank but most corals won't tolerate nitrate.

Another basic measure of reef chemistry is pH. The desirable range for pH is 8.1 – 8.3. The pH in your aquarium will fluctuate based on the time of day. This is known as diurnal fluctuation. One way to help minimize these fluctuations is to use a refugium lit on a reverse photo period from your aquarium. You can learn more about refugiums in this ReefReaders article. Refugiums: What are They and Why in the World Would you Want One.

It is not unusual to hear of people who cannot keep the pH in their aquarium up. One common reason for this is that to CO2 level in the ambient air is too high. To test for this put some water from your aquarium in a container, aerate it for an hour or so, then retest the pH. If is goes up crack a window open to help lower the CO2 level in your house. I have a problem with this in the winter when we use our ventless gas log fire place.

Alkalinity is the measure of the buffering capacity (resistance to change in pH) of your aquarium. Alkalinity should be at 2.5 – 4.0 meq/L or 7 – 11 dKH in order to maintain a stable pH and appropriate calcium level. Both scales are commonly used to measure alkalinity, so you must check carefully to see which your test kit uses.

Calcium is necessary for the growth corals, clams and some types of algae. The calcium level of natural sea water at the surface of the ocean is about 420 ppm. There does not appear that coral skeletal growth is improved when the concentration of calcium is above that of natural sea water1. An appropriate level for a marine aquarium is 380 – 450 ppm.

There is a strong relationship between calcium, alkalinity and magnesium. Calcium and alkalinity are inversely proportional. If your calcium is too low despite dosing check to see if your alkalinity is too high. Another reason that calcium levels will not increase despite dosing is a magnesium level that is too low. Magnesium should be maintained at approximately 3 times the calcium level or 1250 – 1350 ppm.

You should never dose what you do not test for. The first step in testing is to purchase good quality test kits. Test strips are not accurate enough nor are test kits that only give you results like low, good and high. You want to “know what the numbers are” when you test your water. If you are ever having problems in your tank this is one of the first questions you will be asked. If you are having your water tested at the LFS don't accept  them telling you that your parameters are good, ask them what the numbers are.