The pH of your saltwater aquarium is an important aspect of water chemistry. Your briny little swimmers need the pH to hold steady at about 8.2. The level will fall naturally over time as a result of biological processes in your underwater ecosystem, so you'll need to raise it on occasion.
Be Sure Before You Raise the pH
Before taking steps to raise the pH in your marine aquarium, be certain that you actually need to. Always measure your pH at the same time of day; the levels have their peaks and valleys over the course of the day, maxing out in the afternoon. So if you test the pH before dinner today and again before breakfast in two days, you may mistakenly conclude the pH has taken a nosedive. Also, electronic pH meters require calibration—some more often than others—and pH test kits lose accuracy over time. If you haven't calibrated recently or if you've been using the same kit for months, calibrate or pick up a new kit.
Raise It Slowly
While it's true that your saltwater friends and plant life need a consistent pH between 8.0 and 8.4, don't panic if you suddenly find your pH at 7.8. You'll do more damage to your water-dwelling companions by rapidly raising the pH back to 8.2 than by letting them live through a brief period at 7.8, then 7.9, and so on. Marine life has a hard time with big, sudden changes to water chemistry, and they can even be fatal. Some fish are at risk with just a 0.2 pH change at once. So don't try every pH fix at the same time. Gradually raise the pH back up to where it needs to be.
The Airstone Test
The most likely explanation for a dip in pH is an excess of carbon dioxide, usually in the water itself, but sometimes in the air in the room housing your tank. Take a pH reading, then add an airstone to your aquarium. Take another reading the next day (at the same time, of course). If the pH went up, your aquarium does indeed have too much carbon dioxide; if the pH dropped further, the room contains too much carbon dioxide, which is preventing adequate aeration at the water's surface, where it interacts with the air. This can happen in small or closed-off spaces.
Fixing Carbon Dioxide Problems
It's likely the airstone test points to too much carbon dioxide in the water. First, clean out your protein skimmer and make sure it's working right, because it should help keep carbon dioxide levels down unless it's gone screwy. (An upgrade to a more efficient model could help too.) Increase the aeration in your tank to displace more carbon dioxide with more oxygen. Keep the airstone in, or try a bubble wand, fountain, or other aerating device. You could also add a calcium hydroxide solution to the water. Follow the product's directions. If you determine there's too much carbon dioxide in the room where you keep your tank, you have two options: relocate it or increase oxygen circulation in the room by opening a window or using fans to circulate air from the rest of the house.
Sodium bicarbonate—more familiarly known as baking soda—can raise the pH in your saltwater aquarium. Use this method if your airstone test didn't raise or lower the pH and excess carbon dioxide isn't your problem. Start by preparing a partial water change just as you would as part of your normal tank maintenance. Replace one gallon of water for every 20 in the tank. Measure out one teaspoon of baking soda for each replacement gallon and mix it in. Remember, teaspoons are leveled, not the largest mound you can precariously pile up. Over the course of an hour, add in the treated replacement water little by little. If necessary, repeat this pH fix again the next day.
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