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Salinity is a measure of the concentration of dissolved salts in water.
Normal seawater tends to run in the range of 1.021 to 1.025 with the Red Sea
area having a slightly higher level of up to 1.027.
Salinity can be measured in the reef tank using a device called a hydrometer.
This device basically depends on the principal that a higher concentration of
dissolved salts in water causes the water to become more dense and therefore
will cause an object floated on it to displace less water and therefor rise
higher. A typical hydrometer is a glass tube with gradations marked along a
stem. The device is floated in the water to be measured and the level of the
water on the tube is read off the gradations. The same principal is used on
devices which use a floating swing arm to take the measurement. The swing
arm hydrometers are becoming popular due to their ease of use, but many low cost
ones being sold are very difficult to get repeatable, much less accurate
readings.
 |
Glass Hydrometer
This is a typical glass hydrometer. It must be floated in the
tank, which can be difficult if there is much current, or some tank water
must be placed in a deep tube and the hydrometer floated in that.
Accuracy depends upon brand. The one shown has been proven to be
poor and two hydrometers I compared measure .003 difference! which is too
much. |
 |
eSHa Marinomat Hydrometer
Swing Arm Hydrometer. This is a high quality, medium cost unit from
Germany. It is very easy to use as the water is suctioned up into a
reading cavity using a hand compressed bulb and it is temperature
compensated. I highly recommend this hydrometer. |
I recommend that you keep your salinity in the range of 1.024 to 1.025 which
is towards the upper end of the normal salinity range. The higher concentration
of dissolved salts provides a higher concentration of the elements the corals
use for growth. This level also provides some margin of safety if the salinity
level swings up or down a little. High salinity levels of 1.027 may have an
adverse affect on SPS coloration from my observation although I have never seen
this published anywhere before. Another consideration is that if you purchase
most of your specimens from a particular source, you may want to somewhat match
the salinity of your tank to that source to minimize the shock that your new
specimens experience when being introduced to your tank. Large sudden changes in
salinity are probably the number one reason that some specimens, especially
echinoderms, sometimes die within a couple of days of introduction.
Things that affect the tank salinity level after initial setup include:
 | Salts lost through protein skimming effluent. |
| Salts lost due to bagging of specimens or other exporting of the
saltwater. |
| Salts gained through the addition of the saltwater when adding new
specimens (which you should not do). |
| Salts gained through the use of additives used to increase Calcium and
Alkalinity levels, but which may have the indirect effect of slowly
increasing salinity. 2-part additives such as C-Balance and B-Ionic can
cause this. |
| Salinity levels can also change over time depending on how carefully you
mix replacement water. |
Checking salinity should ideally be done weekly and can easily be adjusted
during your weekly water change (if you follow recommendations to make small
frequent water changes). This adjustment is made by adding a little more or less
than the normal amount of salt to the new replacement water. To lower salinity,
you can also just remove some saltwater and replace it with freshwater. To
increase the salinity, you should never dump undissolved salt into the tank.
These salts can irritate or even kill specimens that it comes into contact with.
They should always be predissolved prior to addition to the tank. Lastly, if you
find you need to make a large change in salinity (over about 0.001), you should
do so slowly over the course of a couple of days, to avoid stressing the tank
inhabitants.
Further Reading:
Natural
Reef Salinities and Temperatures
By Dr Shimek
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