Calcium and Alkalinity, pH, Salinity and Magnesium
To the new hobbyist, these terms can be rather intimidating. And when you are trying to consider the relative effects of each on the other, it can become quite confounding. I will try and cover some of the basic elements of the interaction of these elements as it relates to calcium. PH is simply a measure of the relative balance of hydroxide and hydrogen ions in water. In layman’s terms, this means that there are hydroxide elements and hydrogen elements in the water. In a particular solution, whatever the balance is in these two elements determines the “pH”. PH is measured on a scale from 0 to 14, with the midpoint, 7 being described as “neutral”. Solutions with a pH measurement above 7 are considered “base”, while those below 7 are considered “acidic”. Seawater pH is typically 8.0 - 8.5, and is therefore somewhat “base”. Another thing to note about pH, is that the scale is logarithmic. This means is that each gradation represents a factor of ten. So a pH of 8 is 10 times more base than 7, and 9 is 100 times more base than 7, and so on.
What is important about all this is that small changes in pH are very significant to tank inhabitants and to calcium levels. With all things being equal, the higher the pH of the water, the less calcium the water will hold “in solution”. This is not generally a problem as long as the hobbyist keeps the tank pH within the prescribed range, 8.0 - 8.5. However, some methods of adding calcium will affect aquarium pH, which is why it is important to understand the relationship of calcium and pH.
Alkalinity, in short, is the water’s ability to withstand additions of acids, without changing the pH. Other terms that are commonly used to describe alkalinity include buffer or buffering, DKH, meq./ltr., and carbonate hardness. For all practical purposes, they are all the same thing (there are some slight differences in some, but not enough for our purposes here). The elements that provide this effect are carbonates, mostly sodium carbonate and sodium bicarbonate. Alkalinity is necessary in the aquarium to help guard against large swings in pH, which will stress the tank inhabitants. Recommended levels range from 7 dkh to 12 dkh. The other commonly used scale to measure alkalinity is meq./ltr (read as megaequivalents per litre). To convert dkh to meq./ltr you simply divide by 2.8. Relative to calcium, the higher the dkh, the less calcium that will stay in solution. So long as alkalinity is kept in the acceptable range, this should not hinder achieving proper levels of calcium. But, like pH, some calcium additive methods affect alkalinity, thus this basic understanding.
I would also like to refer those who are curious about the calcium/alkalinity dance to an article written by Craig Bingman (Aquarium Frontiers on-line, July 1998). This is an excellent discussion of the relationship between alkalinity and calcium, and recommendations regarding simulating natural seawater levels of both components.
Salinity is another factor in calcium concentrations. The above calcium recommendations are at full strength seawater with a specific gravity of 1.025. If the aquarium is kept at a lower specific gravity, then a given concentration of calcium measured in the water will really be at a higher relative concentration to the other water elements. Put simply, if your tank is run at lower than full strength seawater levels, then the required concentration of calcium will also be proportionally lower.
Magnesium is another player in the dance with calcium. Magnesium levels are typically pretty stable in the aquarium, and are easily maintained with routine water changes. However, if calcium precipitation occurs, or the hobbyist does not do regular water changes, magnesium levels can decline. The recommended level of magnesium in the aquarium is 1300 ppm to 1500 ppm. If magnesium levels do fall, calcium has been found to be much more difficult to get to the proper levels of concentration. However, I want to emphasize that when the hobbyist encounters difficulty maintaining calcium levels, it is best to rule out all other causes before considering magnesium deficiency. This is simply because it is far more often the case that other factors are at play in the problem. If it is suspected that magnesium deficiencies are a problem, it is recommended that a magnesium test kit be purchased and used to verify the problem. Magnesium supplements are readily available, and replenishment can also be accomplished through the use of Epsom Salts.
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