This subject could be a book, but here is a quick summary I wrote. let me know what you think, and any errors. Some of the most common questions we often get asked is, what salt do I use, what do I need to dose, what should my parameters be? In this article, I will try to explain the basics of reef chemistry. For arguments sake, we are going to assume you are using RODI water, maintaining salt water at 1.025-1.026 and have your nitrates & phosphates under control. If not, then you will be constantly fighting problems in your tank. PARAMETERS Alkalinity 2.5-4.0 Meg/L or 7-11 DKH Alkalinity is the hardest principal to understand, but is also one of the most important. Alkalinity is used to determine how much bicarbonate is in the water that is available to corals to secrete their skeletons and grow. The correct range of Alk will also be a buffer to help keep your PH stable. Calcium 375-450 ppm This one is much easier to understand. Calcium is the building block for all stony corals. Without it, your corals will not grow. Magnesium 1100-1400 mg/L This is the third leg of your three legged chemistry stool. Corals also take in Magnesium to grow. The issue with having low magnesium is that when low, your corals will use far more alkalinity and calcium. So if you find yourself struggling to keep your alk & calc up, double check your magnesium level. Alkalinity/Calcium/Magnesium Balance. Think of Alkalinity Calcium, and Magnesium as green, blue and red marbles in a jar. There is enough space in the jar to keep enough green, red & blue marbles at the correct levels. However if you add too much calcium for example, there wont be enough room in the jar to add enough Alkalinity and Magnesium to keep everything in balance. A good goal is to keep all the numbers in the upper half of the range. Iodine .03-.09 mg/L Iodine dosing is a very slippery slope. It is easy to overdose,so beware when testing and dosing. Iodine is removed from the water from skimming & carbon. But it is used by your invertebrates such as shrimp & crabs, as well as for color in your corals. Fish use it to keep a healthy thyroid. Most Iodine levels can be maintained by regular water changes. Strontium 5-12 mg/L The role of Strontium in reef chemistry is not completely understood. What we do know is that it is similar to calcium and magnesium and is used by corals to secrete their skeleton. Unless you have a system that is jam packed with fast growing SPS, regular water changes will keep your strontium level in range. Potassium 350-450 mg/L What really taught me the importance of this component was over time, my reef tank started turning brown and ugly my LPS started to recede. I couldn't figure out what it was, all my parameters tested perfect. I finally send in a water sample to AWT (Aquarium Water Testing). When the test came back, my Potassium was down to 135. I SLOWLY started dosing potassium and within a few weeks my colors came back better then ever. DOSING The most critical thing to remember when dosing is DO IT SLOWLY!!!!! The biggest mistake an aquarist can make is discovering that they have a parameter out of range, and then panicking and dumping a ton of additives in their tank to try and correct the problem. This usually compounds the problem by shocking the system with a sudden change of water chemistry. This can possibly start a chain reaction leading to disaster, so let me repeat, dose & change things SLOWLY. TESTING Remember that testing & test kits are inherently flawed. Since they are usually flawed, I usually try to keep my parameters in the mid to uppersuggested range.