I have grouped these two together, because they are inexplicably linked chemistry wise and need to be considered as a whole. 

A common mistake made in this hobby is monitoring and adjusting calcium levels without paying much attention to alkalinity levels.  This is somewhat understandable given that the need for calcium in the tank, especially a high demand tank is fairly obvious.  What is not so obvious is that the requirement to maintain good alkalinity levels is even higher and the interaction that exists between calcium levels and alkalinity levels inexplicably tied together.  I am a firm believer that alkalinity levels are more critical than calcium levels, so boosting calcium while letting alkalinity do its own thing is not the way to go.  There is also the unfortunate fact that calcium and alkalinity levels tend to counteract each other where increasing calcium forces alkalinity to decrease and vice versa, even if this is not what is desired. 

Normal seawater has a calcium level of about 380 and an alkalinity level of 2.8.  These levels work fine in low and some moderate demand tanks where there is not a lot of calcium consuming inhabitants.  I often see new hobbyists with a couple of mushrooms in a new tank who think that need to be dosing calcium.  That generally is just wasted money and effort.   In tanks that have fast growing calcium consuming critters such as SPS corals and clams, it is generally better to maintain elevated levels of both calcium and alkalinity.  This tends to improve growth, coloration and general health of the specimens.  Calcium levels in the range of 400 to 450 are preferred with up to 500 being acceptable. Alkalinity levels of 3.2 to 4.5 is preferred.  Some have reported good results using the more normal 2.5 to 2.8 alkalinity level of normal saltwater which is what most salt mixes mix up to, but I have had my SPS corals tend to brown out under those conditions.

 Water changes alone are sufficient to maintain normal seawater levels (depending on what level the salt mix provides) in low and some moderate demand tanks.  In order to elevate the levels or even maintain normal seawater levels in high demand tanks requires that the levels be supplemented somehow.  This is where it can get to be a little tricky.  There are five basic ways to accomplish this task

 Kalkwasser 

Kalk is the oldest and one of the still commonly used means to maintain calcium levels and alkalinity.  Kalk is a dry powered form of lye that is mixed with water to create a solution rich in calcium that has a very high pH.  This solution is allowed to settle and the clear portion of the solution is siphoned off and dripped into the sump slowly so as not to upset the pH of the tank water.  This is usually dipped in overnight when pH of the tank tends to be lowest.

 Many experienced reef keepers swear by the use of Kalk.  My results have been less than satisfactory and I don’t recommend it for several reasons. 

 The main good thing about Kalk is it is relatively cheap compared to most other options.  There is also evidence that Kalk can help to participate phosphates out of the water. 

If you want to try Kalk, go for it, but keep a close eye on your tank readings.  I won’t attempt to give a formula to use, because I was never very successful using it, so I obviously don’t know what the correct formula is.

 
2 Part Liquid Supplements 

There are two common 2-part supplements on the market.  B-Ionic and C-Balance.  These both come as liquid additives in 2 parts (hence the name).  One part is to add calcium and trace elements, the other part is a buffer/alkalinity component.  These additives seem to work fairly well in low to moderate demand tanks where they are used to maintain existing levels.  In high demand tanks, they sometimes seem unable to handle the demand and the levels fall, even when large quantities of the additives greater than the recommended maximum are used.   As the quantity is increased, there can be a tendency for these supplements to cause the sand to clump and form a solid rock like structure which is not a desirable thing to have happen.  2-part supplements elevate salinity, so when using large quantities, you will need to keep an eye on the salinity and adjust it occasionally.  The other downside to the 2-part supplements is that the cost can get very expensive when you are going through it fast.  I recommend 2-part supplements in low to moderate demand tanks where the amount required is moderate and you are only trying to maintain water parameters and not adjust them.  They are probably the easiest supplement to use.

 Liquid Calcium Supplements

 There are a number of liquid calcium products on the market.  Most have names like ‘turbo’ or similar.  The main issue with these products are that they do not take alkalinity into account and a common mistake is to measure calcium only and using one of these products to adjust to the level you want without realizing the alkalinity levels are dropping.  Simple fact is that alkalinity level is more critical than the calcium level.  Usage of these products requires that you monitor alkalinity as well and use a buffering product to maintain alkalinity levels.

Dry Calcium/Alkalinity products

 These are what I used to use in my high demand tank.  Using these products, I was able to adjust and maintain whatever levels I wanted to without clumping the sand or having the pH get out of whack, so I have become a big fan of them and recommend them highly. 

I used 'Reef Builder' to maintain alkalinity and 'Reef Advantage' for Calcium, magnesium and strontium supplementation.

Being in dry powder form, they need to be dissolved in water before adding to the tank.  I personally mix my buffer in the AM and drip it in before I go to work.  I mix the calcium component and drip it in before I go to bed at night.  The addition of the two components should be separated by at least 30 minutes per the Seachem literature, probably to prevent precipitation.  I drip it into where my tank overflow is in the sump so that it gets quickly mixed into the water.    My own 70 gallon high demand tank required about 7tsp. of Reef Builder and 5tsp. of Reef Advantage each day to maintain my Alkalinity around 4 meq/L and calcium around 450ppm.  Obviously the amount appropriate for any particular tank must be determined through testing of the water.  

Calcium Reactor 

Calcium reactors are becoming very popular on high demand systems.  Although they require some dialing in, once setup they are relatively low maintenance.  You no longer have to mix supplements by hand every day and since they run continually, the tank parameters will be maintained at a more consistent level than once a day additions can achieve.  It also has the benefit that it will not cause calcium deposits on pumps, etc. as the other methods can.

So what’s the downside?  Primarily cost.  Calcium Reactor systems require a reactor chamber, a pressure regulator and a compressed air tank, plus CO2, aragonite media, a bubble counter to see how fast the CO2 is being dispensed, etc.  A typical system will run about $500 - $600.  Of course, the other additives tend to be fairly expensive over time whereas the calcium reactor is more of a one-time hit.  The other potential negative to a calcium reactor is that it can depress the pH level of the tank.  This can generally be controlled fairly easily.

I would highly recommend a calcium reactor for high demand SPS tanks.  It may be overkill for low to moderate demand tanks.