pH probe

What is pH?

by Brian Dunat

When you setup your first saltwater tank, you probably got the basic run down from your local fish store on how to keep track of the major tank parameters, light, salinity, and temperature.  They probably also told you how to test and monitor each of these, but what about pH?  Why is it important?  How do you control it?  Should you try to control it?  Why does it vary from day to night? These are all questions most marine hobbyists have had at one time or another and we’re going to dive headfirst and take a closer look at pH.

pH in the Marine Tank

Before we begin, we really need to understand what pH is and what impacts it. pH, in simple terms, is the measure of H+ ( Hydrogen ions) in the water. The more of these H+ ions, the lower the pH is. Within the saltwater tanks, the chemistry of pH is important in 2 primary areas, photosynthesis and calcification.  In photosynthesis, algae in our corals’ tissue take light and CO2 and convert this to sugar and oxygen.  This results in a net increase in the pH during the daytime in our tanks.  At night, the reverse process of Respiration consumes the Oxygen and releases CO2, lowering the pH and making the water more acidic.

Calcification and pH are related as well in our reef tanks.  Corals take the energy of photosynthesis that is produced in their tissues and use it to take Calcium out of the water to make their skeletons. In this process, the surrounding pH conditions are important as the more acidic the water, the harder it is for the coral to build up its skeleton.  Ironically, those of us that use calcium reactors are in effect running this process in reverse.  Adding CO2 to make the water in our calcium reactor more acidic, which in turn dissolves the deal coral skeletons and releases all the ions we need in the aquarium ( calcium, alkalinity, magnesium, trace elements).

Test kit for testing phosphates in your aquarium

What Range of pH is best?

Now that we understand what pH is and how lighting  is one of the key elements to controlling CO2, we need to discuss what is a normal pH range for a marine tank. Typically, pH ranges from 7.8 at the lowest up to
8.5 with the sweet spot being  8.2- 8.4. Anything below 7.8 or above 8.4 can be troublesome to some of the more difficult to keep SPS corals.

pH and Alkalinity

We would be remiss to talk about pH without including alkalinity in the conversation as well.  Alkalinity in seawater is made up of many “things”  that sum up as result.  That result is actually what we call Alkalinity.  Scientifically, it is the measure of a solutions ability to buffer against a change in pH.  In sea water the majority of this buffer comes in the form of bicarbonate ions (HCO3–) and carbonate (HCO3-) which corals take up to make their stony skeletons as carbonate (CO3–).  So basically when we test our Alkalinity each week, we are testing the amount of  bicarbonate that exists in our reef tanks.  Typically this is measured in DKH or in MQ/L and should be kept as steady as possible.  Many a reef melt down can be attributed to alkalinity swinging up or down to rapidly as well as it being too high or low.  A rule of thumb is to keep your 24hr change of alkalinity below 1.4dkH if you are trying to raise of lower it to make sure you don’t shock your corals.  In general, we aim for a dKH in our tanks of  8-11 in order to keep all the corals happy.

Here is a picture of our coral quarantine tank

Controlling pH

So now that we have talked about what pH is, what alkalinity is, and why the two are important, how are they related?  How do you control them?   For years, I have tried to think of a good visual on how to explain the relationship between alkalinity and pH and here is the best i’ve got so far..  pH  in the reef aquarium should be treated like the temperature in your house.  It can be high or low, but no matter if its summer or winter, right around 68-70 should be a good value.  Alkalinity comes in to this picture as your heat/ac.  With in reason, no matter what the temperature outside, your heat/ac is able to buffer the inside temperature to stay in that 68-70 range.  In the reef tank world our “heater/ac”  is typically our top off water or dosing pump which adds thinks like bicarbonate water or kalkwasser to our tanks.  Both of which result in adding bicarbonate and thus buffering the pH from swinging.

Controlling the addition of kalkwasser/bicarbonate solution is easily done with a reef controller like those offerred by Digital Aquatics and Neptune.  These monitor your pH and then can take various action steps to raise or lower your pH depending on how you have it setup.  A swing of .2 – .4 between day and night is normal and if you time your kalkwasser addition or dosing to be during times of lower pH when the lights are off, it can even help narrow this pH swing further. This type of a swing is generally tolerated by the fish and corals as it is probably similar to what occurs in nature.  The rule of  thumb with pH in our tanks should be like every other important parameter; RAPID CHANGE IS BAD!   Again, ” Nothing good ever happens quickly in a reef tank”.

As we discussed earlier, lighting has a large impact on the pH in our tanks.  If you have a sump with chaeto and a light on it, you can further reduce the pH swing of your tank by keeping this light on an opposite schedule from your main tank.  During the day, one will do photosynthesis, and during the night, the other one will and this will cancel out the pH .  For example, if you run your main display tank lights from 10am – 8pm, then try running your sump light from 6pm – 10am. This is by far the easiest adjustment to make and generally leads to the biggest improvement.

What happens if the pH still has a large swing or is too low? Another issue that we’ve had recently with our tanks is related to low pH. To address this issue, we adjusted the lighting schedule, increased our chaeto volume in the sump and still no change. In discussions with our LFS owner, Don Mikos at Sea Escapes, we came to a conclusion that we may need to oxygenate the water more to properly dispose of the surplus of CO2.

This brings us back to a basic problem where some people will install glass lids on tanks which essentially keeps fish like wrasses from jumping but also traps CO2 within the tank. To address this issue, we installed a fan on top of one of our frag tank racks that doesn’t get a lot of normal circulation and also pointed a power head up towards the surface of the water to generate some waves. Within a week, we saw the pH naturally rise from 7.8 to 8.1, and we also reduced our pH swing from .4 daily to .2 pH swing during the night hours by adjusting the lighting schedule on our sump.

All in all, these were simple changes we were able to make without the need to add anymore chemistry to our tank.

Some other useful things to check do are:

1. Make sure to recalibrate your pH probe often.  Using the calibration solution, this process should take no more than 10 minutes to complete on a monthly basis and will ensure you are getting accurate pH measurements and not reacting to a mis-calibrated probe.

2. Prior to doing a water change, make sure the mixed salt water chemical levels are in line with what your tank is currently running at. If not, adjust the levels in the new water before you do your water change as this can also inadvertently affect pH if chemical levels are off.

Knowing a little about your water chemistry is crucial to keeping a successful reef tank.  Check your levels often, make your changes slowly, and remember, NOTHING GOOD EVER HAPPENS QUICKLY IN A REEF TANK!  🙂

Have you found other solutions to stabilizing pH levels and reducing the pH swings? We’d love to hear about them.


Brian Dunat

Brian grew up in the Midwest and has always enjoyed interacting with the natural environment in some way, shape or form. From a young age, snorkeling in the Eau Claire Lakes in Wisconsin to his mid 20’s when he became certified diver, Brian has always enjoyed spending time underwater. Brian started out with a lot of fresh water tanks that just seemed to keep getting bigger and bigger. Shortly after purchasing his home in 2003, he was in his LFS and decided to invest in a “small” 40 gallon saltwater tank.  10 years ago, he had no idea he would end up with hundreds of gallons and many species of corals growing happily and sustainably.


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