Water chemistry is a truly loaded topic, so we will try to simplify it as much as possible. The quick and easy answer to maintaining high water quality is to do regular water changes and periodically test the water for various compounds. If you keep up a consistent maintenance schedule, the reef tank should be fine. What actually goes on behind the scenes is dizzying in its complexity.
The chemistry of a reef aquarium constantly changes. Corals uptake various compounds from the water that need to be replenished. Water evaporates making the tank saltier. The buildup of certain compounds can create ionic imbalances that impede coral growth and development. With this in mind, there are four major parameters of water chemistry that should be periodically measured:
Salinity is the concentration of salt in the water. To actually find the salinity, you would have to do a very costly laboratory grade titration test. Instead of directly measuring the salinity of the water in our tanks, we measure the specific gravity which closely relates to the salinity of the water. The most popular method of measuring specific gravity is through the use of a hydrometer or a refractometer.
|Hydrometers come in a few forms, but are basically a small container with a floating arm. The buoyancy of the swing arm tells you the density of the water. Hydrometers are inexpensive, but also very inaccurate. The swing arm may come in contact with bubbles, and as a result give you a higher salinity reading. The hinge may also get gummed up over time further throwing off the measurement.|
|Refractometers measure specific gravity using the refractive index of the water. We recommend them despite their higher cost since there is much less that can go wrong with the measurement. Salinity is not an insignificant measurement, and we should strive to get as accurate and consistent a reading as possible.|
The average salinity of the world's oceans is 35 PPT which equates to a specific gravity of 1.025-1.026.
Reef building corals require a supply of calcium and bicarbonate in order to make a skeleton and grow. Test kits for calcium and alkalinity are readily available, and should be standard fare for anyone attempting to keep stony corals.
pH is a measure of the concentration of hydrogen ions (H+). It's basically an indication of how acidic or basic the water is. If there's a high concentration of H+, then the pH is low (<7.0), and the water is acidic. If the concentration of H+ is low, then the pH is high (>7.0) and the water is alkaline (also called basic).
Alkalinity is the ability of the water to resist a change in pH. If the alkalinity is high, then the water has a greater ability to buffer the water, and so resist rapid changes in pH. Chemically, alkalinity is made up of the anions (negatively charged particles) in the water. Since carbonate and bicarbonate are two of the major anions in an aquarium, carbonate hardness (dKH) is often used to estimate alkalinity. When H+ (acid) is added to the tank , it will combine with the anions, and use up some of the alkalinity. When those anions are used up, any additional H+ added to the tank will start affecting the pH.
Here's the common problem, the term "alkaline" in reference to pH is not related to the term "alkalinity." The pH and alkalinity affect each other, but the terms are referring to separate parameters.