Example #1: With calcium at 300ppm and alkalinity at 11 dkh.
Example #2: With calcium at 340ppm and alkalinity at 8dkh.
Very good point, lets look at the difference in these 2 test results. They look similar, but they tell a totally different story.
The safe range that we want for calcium is 360ppm to 500ppm, with a target of 400ppm to 460ppm. We are trying to stay in the target range, but we won't cause serious problems if we are in the safe range.
The safe range for alkalinity is 8 to 14 DKH, with a target of 10 to 12 DKH. We prefer to stay in the target range, but won't get to worried so long as we are in the safe range.
Your goal is to get both calcium and alkalinity inside the target range at the same time, and then develop a buffering and supplementation routine that keeps it there. You will do this by testing frequently until you find a pattern, at which point you can begin to test weekly. Every time you add new livestock this will change slightly, as the bioload increases. Once the tank is fully stocked, you will eventually just be doing the test every week, unless you see something out of the ordinary.
Calcium drives alkalinity to a degree. Calcium will drop, then alkalinity will follow. If alkalinity is low, you have an absence of multiple buffering ions, including calcium carbonate, but also including bicarbonate buffers, borate, magnesium, and others. Calcium Carbonate is the primary buffer.
If you attempt to rains alkalinity and the calcium level is not where it needs to be, then alkalinity tends to drop back down quickly.
In Example #1, your alkalinity was within the target range and calcium was low. Adding a buffer to increase alkalinity would not have increased the other buffering ions at an improper ratio with calcium. We first needed to increase calcium into the correct target range.
In Example #2, both calcium and alkalinity are in the bottom of the safe range. You had been adding calcium, and it was depleting quickly. This was because the other buffering ions are also low, as indicated by the alkalinity test result. You need to add both calcium chloride and the super buffer product to move both calcium and alkalinity up into the target range.
Notice that Example #2 is a better place to be than #1. In #2 you have both alkalinity and calcium moving together on the scale, and indication that the buffering ions are at a balanced ratio. You are only 1 or 2 doses of both away from having each inside the target zone, at which case it will be easy to maintain.
There is one more variable to this, which is coraline algae. Coraline aglae takes up calcium from the water. So at times you may find that you need to dose Calcium Chloride and not the buffer. When you see calcium on the low end and alkalinity in the target range, then this is when you add Calcium chloride by itself, without adding the buffer.
Don't feel bad. This is a crazy complicated topic that very few people understand. I don't have a clue about the WHY behind the chemistry, but I have learned the WHAT to do to keep it safe, which is all that matters for our purposes in this hobby. If you want a real high level discussion on alkalinity, I suggest that you google "Randy Holmes Farley". He is the guru and his methods are where I have adapted my methods from.
For a more simple long version, I posted an article that might provide more help: http://www.tropicalfishkeeping.com/m...-marine-33079/