Aquarium Water Chemistry

This is a simple guideline outlining water chemistry parameters for reef tank type aquariums. When we set up our first aquarium, we read many books and articles on water chemistry and honestly most of what we read was confusing and misleading. For the first year we had a big chart up on the wall listing all the supplements we needed to add, when we needed to add them, what tests needed to be done and when. It wasn't until we removed that chart and focused on the basics that we started to really enjoy the hobby and truly understand how aquarium water chemistry worked. We still have that chart as a reminder of what not to do .

The composition of seawater is complex, but keeping your water chemistry at a level needed to keep marine fish and invertebrates healthy and growing need not be. For a more detailed explanation of the composition of sea water see here http://reefkeeping.com/issues/2005-11/rhf/index.php



The first main parameter we should be worried about is salinity/specific gravity.


On average NSW (Natural Sea Water) has a salinity of 35ppt, or 1.026 SG.
This world map shows how the salinity of the oceans changes slightly from around 32ppt (3.2%) to 40ppt (4.0%). Low salinity is found in cold seas, particularly during the summer season when ice melts. High salinity is found in the ocean 'deserts' in a band coinciding with the continental deserts. Due to cool dry air descending and warming up, these desert zones have very little rainfall, and high evaporation. The Red Sea located in the desert region but almost completely closed, shows the highest salinity of all (40ppt) but the Mediterranean Sea follows as a close second (38ppt). Lowest salinity is found in the upper reaches of the Baltic Sea (0.5%). The Dead Sea is 24% saline, containing mainly magnesium chloride MgCl2. Shallow coastal areas are 2.6-3.0% saline and estuaries 0-3%.







The next parameter we should worry about is actually 3 parameters. Calcium, Alkalinity (bicarbonate) and Magnesium.


I'm including these three together because they all work in relation to one another and in part can have an effect on each other.

Calcium is found in a concentration of about 420ppm. For us an acceptable range would be between 380ppm to 450ppm.

Magnesium is found in a concentration of about 1290ppm. For us an acceptable range would be between 1150ppm to 1350ppm or roughly 3 times your calcium concentration.

Alkalinity (bicarbonate) is found in a concentration of about 125ppm(mg/l= meq/l) or 7dkh. For us an acceptable range would be between 125ppm/ 7dkh to 215ppm/ 12dkh. With alkalinity it’s a good idea to keep your levels a little higher then NSW levels(125ppm) because alkalinity can be depleted quickly and if it falls below NSW levels can have detrimental effects on calcification and allow large swings in PH.


For a more detailed explanation of the relationship between Calcium, Magnesium and Alkalinity please see http://www.reefkeeping.com/issues/2006-06/rhf/index.php



Next- PH.








This world map of ocean acidity shows that ocean pH varies from about 7.90 to 8.20 but along the coast one may find much larger variations from 7.3 inside deep estuaries to 8.6 in productive coastal plankton blooms and 9.5 in tide pools. The map shows that pH is lowest in the most productive regions where upwellings occur. It is thought that the average acidity of the oceans decreased from 8.25 to 8.14 since the advent of fossil fuel (Jacobson M Z, 2005).

PH can be confusing and is often over stressed. In sea water, PH is controlled for the most part by CO2. The more CO2 in the water and in the surrounding air the lower the PH will be. If we keep our Alk, Ca and Mg in the acceptable levels as out lined above our PH should never rise or fall out of acceptable levels. An acceptable level for PH is anywhere between 7.8 and 8.6. One of the biggest problems I see people have with their aquariums is that they try to chase an Ideal PH number.