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Adjusting Saltwater for a Water Change

In the last post, “How to Mix Saltwater,” we discussed how it is best to adjust the aquarium water and the new saltwater for a water change individually rather than after a water change.  Some salts may be low in alkalinity, calcium, or magnesium, and using that saltwater on a thriving reef tank may be detrimental. If you don’t test these parameters on each batch of new saltwater, at least test them when using a new box/bag/bucket of salt.

In this article we’ll discuss how to check the water before use.  Again, this testing is done after the saltwater has mixed for at least 24 hours to allow all the salts to dissolve and for the water to aerate.  Testing prior to the 24 hour mark may result in low numbers.

Saltwater Mixing

Saltwater Mixing

Salinity

Step 12 in the article mentioned that the salinity of the new saltwater should match the salinity of the reef tank (unless, of course, you are trying to raise or lower the salinity of your reef tank.)  For instance, if you run an SPS-dominant reef, you probably have a salinity around 1.026.  Therefore, you would want your new saltwater to also measure at 1.026.  If, through heavy skimming or other means, your reef tank salinity dropped to 1.024, and you needed to raise it back to 1.026, you would make higher salinity saltwater.  I like this Target Salinity Calculator (external website.)

Temperature

Step 13 discussed matching the two water temperatures.  Make sure to use the same thermometer on both the tank and the new saltwater as there can be variations between measurement devices.  For instance, I run a Neptune Apex controller on my main display with a temperature probe.  The accuracy of the probe probably does not match the $2 USD thermometer I use for water changes.  Since I run my reef tank at 76 degrees Fahrenheit, I try use replacement saltwater that is 76°F as well.  This is especially important if you are doing more than a 10% water change.  In all honestly, if you are only doing a 10% water change, temperature is not critical within normal reef parameters (76-80°F), but using water within one degree is best.  I do not recommend replacement water that is below 75°F or above 80°F. for a typical reef tank as this may stress inhabitants.  Obviously, if you are running a cold water tank, then reef tank temperatures do not apply.

pH

At Step 14 we get into the more complicated testing.  First up is pH.  The easiest way to adjust this is to aerate the new saltwater for 24 hours beside the aquarium.  According to Dr. Randy Holmes-Farley, the target pH is 8.2, but an acceptable range is 7.8-8.5 (Link to his article on low pH).  I’d refer to his article for a more-than-detailed description on why pH is important to a reef tank.  Again, use the same measurement device for the aquarium as the new saltwater, and I recommend using a meter rather than a test kit due to accuracy issues.  Make sure the meter is calibrated, and if it is an inexpensive meter, you can calibrate it with this DIY pH Probe Calibration Solution.

Calibrating a pH Probe

Calibrating a pH Probe

If the pH is too low (very common in newer homes due to better insulation and therefore more carbon dioxide), you can try aerating it longer with a window open near the mixing container.  I’ve even placed an air pump outside the window temporarily (weather permitting) and ran an air line to the container.

Battery-Operated Air Pump

Battery-Operated Air Pump

If aeration doesn’t work, it may be an alkalinity issue.  pH buffers can be used, but they are usually just a short-term fix and may cause more problems with alkalinity (even though many manufacturers tout that their products will not affect alkalinity.)

Reef pH Buffer

Reef pH Buffer

High pH problems are typically due to a lack of carbon dioxide, and aeration typically works here as well.  Both soda water and vinegar will reduce the pH level, but I recommend just aeration for the most simple and safe fix.  In extreme cases, Dr. Holmes Farley recommends using 6 ml soda water or 1 ml vinegar per gallon of saltwater to reduce the pH by 0.3 (link to article).

Vinegar

Vinegar

Alkalinity

Which brings us to testing alkalinity!  I use the Red Sea test kit to make sure my aquarium alkalinity matches the new saltwater alkalinity.  I try to keep my alkalinity at 10 dKh (but this will vary from reefkeeper to reefkeeper.)  If I find that the alkalinity in my aquarium is 8 dKh, but my new saltwater is at 10 dKh, then I will use this handy calculator (external website) to calculate how much alkalinity product I need to add in order to raise it to 10 dKh.  Once I’ve added the product to raise the alkalinity, I wait at least an hour for it to really mix in.  I also retest the pH as alkalinity changes will typically affect the pH.

Red Sea Alkalinity Test

Red Sea Alkalinity Test

Calcium

Again, I use the Red Sea test kit to make sure the calcium is correct in my reef tank and that the new saltwater matches the reef tank.  I try to shoot for about 450 ppm calcium.  I use the reef calculator mentioned above to help determine how much to supplement.  The calcium affect on pH is generally minimal.  If the calcium is too high in the new saltwater (up to 500 ppm), I still use it.  There will be some precipitation, but it’s not too bad.

Red Sea Calcium Test

Red Sea Calcium Test

Magnesium

Lastly, I check magnesium with the Red Sea test kit.  1350 ppm is my magnesium goal in both my reef tank and new saltwater.  If one is off, I use the reef calculator to bring it up to the correct level.  If the magnesium is too high in the new saltwater (up to 1600 ppm), I use it anyway as the reef inhabitants will use it.

Red Sea Magnesium Test

Red Sea Magnesium Test

Last Thoughts

I recommend testing/adjusting the salinity, temperature, and pH before testing alkalinity, calcium, and magnesium.  Then, I recommend testing alkalinity, calcium, and magnesium before correcting those parameters as they all play an important role together.  Although this sounds like a lot of work, it is a crucial step in ensuring a reef tank is as stable as possible.

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