I'll also take a stab at an explanation of alkalinity. In nature water will pick up dissolved minerals and one of the largest rock building mineral families is the Carbonates. These minerals are identified in their chemical formulas by the CO3
( the 3 should be written as a subscript ). These carbonate compounds will react with acids in a neutralization reaction.
As the biological processes occur in your tank, many of the waste products are acids. These acids bring your pH levels down. These acids will react with whatever carbonate ions are available, however, after a certain point there will be no more carbonate ions left in the water ( this occurs at about pH of 4.2) and additional acids produced by the critters in the tank will cause a rapid pH drop. By adding buffers to your tank, you provide the means for the water to resist a change in the pH by providing the carbonates with which they will react ( the neutralization reaction ). So in this since having a large alkilinity should be a good thing.
Now if I understand the rest of the chemistry, the problem occurs when the alkalinity is too high because the calcium ions, which are in solution in the saltwater ( and which the corals and such use for their skeletons), will react with the carbonate ions and precipitate out. This removes the usable calcium from the water column.
Think of it as trying to find a suitable date. If you ( the calcium ion) go to a club with only a few other patrons( carbonates) , your chances of "hooking up" are low. If however, you go to a club that it packed with people ( the carbonates) you will be more likely to bump into someone you can hang with.
When the tank becomes overcrowded with carbonates ( alkalinity too high), there are far too many opportunities for the calcium ions to link up and precip out. Therefore high alkalinty and high calcium levels are mutually exclusive.
How did I do?