The aquarium salt mix/solution
Each conventional salt mix/solution has its own proprietary calcium content and it is a know fact that certain brands are considered better than the others. Since calcium can be in the form of calcium carbonate, calcium chloride, calcium acetate or calcium hydroxide, do not assume that all of them are the same as they have will have their own solubility in water and interactions with the other chemicals will affect pH and other parameters. The most commonly used in the reef tank community is the Kalkwasser which is a solution of concentrated calcium hydroxide slowly dripped into the system and so far this is one which proves to be working well. In order words, don’t buy into the idea that all salt mixes are the same as some of it can actually result in calcium deficiency occurring in your tank. Usually the best mixes should be a balanced "two-part calcium and alkalinity" supplements working together more like a buffer system so that when you increase you calcium content, this will not cause a major shift in your pH level. Most common are either the B-ionic or C-Balance sold in pet stores specializing in marine tank equipment and test solutions and there are other brands like Seachem and Kent supplements worth mentioning here as well.
The method on how you add your Calcium
How you add your supplement will also affect the concentration and also on whether the calcium is put to good use available for your stony corals. For example, the reason why Kalkwasser has to be slowly added to the water is because it needs ample time to react with the available carbon dioxide or else you will start to see precipitation of white grains of calcium carbonate forming everywhere from the heater to the tank filter. There is another method recommended by aquarist involving the use of vinegar to mix with the Kalkwasser solution prior to adding to your saltwater in order to avoid the precipitation problem but by doing these, come with associated risks as most vinegar has unwanted impurities in it which means that you are introducing these unwanted stuff to your fish tank. Second, leftover acetate can actually promote bacterial growth, although to certain extent, that is considered good but somehow you can’t control which type of bacteria will propagate as it will instead ended up feeding those bad bacteria which will cause more problems to your fish and invertebrates.
Problems coming from pH adjustment
Alkalinity will affect how much calcium is available in your reef tank, in other words, both of them are actually inter related. Therefore, in order to ensure that there is enough calcium in your reef tank, you will also need the right alkalinity level meaning that you need to strike a right balance between those two. Most problems actually started when alkalinity is already at the higher end and the aquarists will keep on adding alkaline booster in order to correct the problem but in actual fact is causing other issues as the calcium level will drop. This is actually a common scenario because most people do not realize on what is the right alkalinity level to maintain and somehow there is also confusion between the difference of meq/L and dKH. The normal level often seen is usually alkalinity at 3 -4 meq/L or 8 – 11dKH and no more of the alkalinity booster is needed when the alkalinity already falls within this range. In order to bring up calcium level, you should instead use dry calcium chloride adding in small amounts to make the adjustment. If this doesn’t work, then the only solution left is to conduct a water change.
Inaccurate aquarium test kit reading
Finally, last but not least, do not believe in whatever reading that you get. Sometimes, despite doing the correct adjustment you will still ended up with weird readings and that is the time that you will need to do some cross checking using other test kits. The fastest way is to actually ask from help from a fellow aquarist to get them to lend you a set so that you can quickly confirm whether your reading is accurate rather than buying a whole new box just to conduct the test. All items have its own shelf life and that includes test kits as well, thus check with the supplier what is the validity date on the use of these items. At times, read also the instruction for use of these test kits as improper handling or incorrect methods can also result in wrong reading given. In worst case situation, it was often a scenario that has a healthy natural reef system turned into complete disaster because of wrong information.