AMMONIA
There are 2 types of ammonia - free ammonia and ammonium. Free ammonia is what burns the fish's gills. Fish will come to the surface and gulp air when the ammonia is high. Ammonium is always present and is harmless. Most test kits will show a minimum amount of ammonia and what you're reading is actually ammonium. Once your tank is established, your test should read zero or a trace depending on the test kit and further testing is unnecessary. Sources of ammonia are dead corals, dead fish and OVER FEEDING.

NITRITE
Nitrite is formed when ammonia absorbs oxygen - hence lots of water surface movement is required. For each ammonia molecule, 2 oxygen atoms are added and 3 hydrogen (or acid ions) are released - this is why pH drops during tank cycling. Nitrite is poisonous to fish as is ammonia. Once your tank is established, your test should read zero and further testing is unnecessary. If nitrites are present, then ammonia was there for the same reasons.

NITRATE
Nitrate is formed when nitrite absorbs one more oxygen atom. This is the final product of ammonia break down. Nitrates are relatively harmless. Fish can tolerate nitrates over 100 ppm but they are subject to nitrate burn. Corals like zero nitrates but under 20 ppm are fine. Plenums reduce nitrates to zero and are highly recommended. Monitor nitrates especially if you don't have a plenum.  Deep sand beds simply do not work.
NOTE:  REEF VITAL DNA BY MARC WEISS WHEN ADDED TO YOUR TANK, WILL GIVE A FALSE POSITIVE TEST FOR NITRATES.

pH
pH is a measurement of how acidic or basic your water is. Saltwater aquariums should test an average of 8.3 to 8.4. Your highest reading is at the end of the lighting cycle, about 8.6, and lowest when your lights turn on, about 8.0. This is because of CO2 being used (pH rises) while the lights are on (photosynthesis) and CO2 absorption into the water at night (pH drops). Fish respiration is a large contributor of CO2.

ALKALINITY
Alkalinity, in simple terms, is your overall water quality. Comprised of a complex buffering system of carbonates and bicarbonates, average seawater is about 175 ppm or 3.6 Meq/L. An aquarium with a reading of 360 ppm or 7.2 Meq/L is good for reef tanks. Raising alkalinity levels too high will lower your calcium levels.

PHOSPHATES
Phosphates come in several different forms and because of this, only certain types can actually be tested for. Fish food is the primary source for phosphates. Phosphates are required for living organisms to exist. Therefore removing phosphates completely is not recommended nor is it even possible. They do contribute to algae growth when combined with nitrates.  BLUE LIFE now makes a Phosphate control that is a liquid and remarkable.  We have reef tested it at 4x overdose and it's completely safe.  Much easier to used that a phosphate reactor.

CALCIUM
Calcium is essential for many things. Corals with a skeleton, halameda algae, coralline algae, snails, crabs, shrimp to name a few. Calcium levels are really not critical. Anything between 350 and 450 ppm is fine. If you're adding a calcium supplement and your tests read consistent and your corals look good, then don't try to push your calcium level higher. Adding alkalinity boosters will push calcium levels down and adding too much calcium will push your alkalinity levels down. Careful!
Kalkwasser or Calcium hydroxide is not a very good calcium additive. When first mixed with water the pH is over 13.5. Drain cleaner has a pH of 14. After the solution settles is tests about 12. Adding kalkwasser too quickly raises pH and alkalinity and actually decreases calcium by precipitation. If you calculate the amount of calcium you're adding per liter, it's pretty low (600 mg/l) compared to other additives such as Sea Chem's Reef Calcium (50,000 mg/l) and a lot more work.
We carry a compressed block of calcium and 27 other minerals by SeaLab called SeaLab 28. This is much better than using a calcium reactor or adding daily or weekly supplements. You put the block in a couple filter bags and place it in a quite spot in the sump. Over time it raises calcium levels to 450 ppm and stays there until the block dissolves. Smaller SeaLab blocks are available for tanks without sumps.

STRONTIUM
Strontium is the element used in fireworks for red colors. It is also the reason coralline algae is red. Testing is not important. If your coralline algae starts to bleach, your strontium is low. Add more strontium and coralline growth increases.

IODINE
Iodine is a necessary element in all reef tanks. Creatures that molt, like crabs and shrimp, especially need iodine. Xenia loves iodine too. Iodine in your tank is chemically known as iodide. Iodine is actually poisonous, iodide is not. Lugol's Solution is a fancy type of tincture of iodine and when you add it to saltwater, it loses its color as it changes from iodine to iodide. You have just spent a lot of money on what ends up to be sodium iodide - an additive of table salt which is cheap.

MAGNESIUM, MOLYBDENUM AND OTHER ELEMENTS
There are many elements in sea water - probably around 80 - most are in very tiny amounts. These aren't worth worrying about. Salt mixes have all the important things. Water changes will handle these elements.
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By the way, salt mixes also include chlorine removers. It isn't necessary to dechlorinate your water unless it is exceptionally bad. If you use RO or DI water, do not add dechlorinator.