How To Raise pH on a Reef Tank Safely

What Is The Best pH for Reef Tanks

In order to understand what is the best pH for a reef tank, we’ll need to look a bit into how coral grows, as that process is heavily influenced by the pH and hardness of the water. A coral reef in nature is a colony of polyps, really small organisms that build up an external hard skeleton for protection. This skeleton is what people usually call coral, and the skeletons of polyps make up a reef. Calcium availability, water hardness and pH are three very closely related factors that affect this process.

In order to build up a coral reef, the polyps use bicarbonate ions and calcium ions to create deposits of calcium carbonate (CaCO3 or aragonite), using energy they obtain from light through photosynthesis. This process is known as calcification and it’s more efficient at high pHs, becoming more and more inhibited the lower the pH goes until a point when it just won’t happen (hence why acidification of oceans is a serious issue for corals). So, in order for your coral reef aquarium to thrive and grow you will need intense light in the correct spectrum, calcium and bicarbonate ions (Ca2+ and HCO3-).

Reef aquarium in Monaco
Some tropical corals can deposit enough aragonite to grow over a cm per month!

Ocean water has a pH around 8.2, but the pH on a reef tank can deviate from that significantly at least during some parts of the day, and most reef aquariums can thrive in pHs as low as 7.8 as long as there is extra calcification nutrients available. A carbonate alkalinity of more than 3 meq/L, usually achieved through calcium carbonate/carbon dioxide reactors, provides the coral organisms with enough building material to calcify even if the calcification process is less efficient at lower pHs where getting enough carbonate to deposit is an issue.

In a lower pH setting calcium levels over 400ppm will allow your coral reef to still get enough to grow, despite the lower pH, though ideally you would still aim to increase the pH on your reef tank to closer to 8. However, be careful not to increase it too much as once you surpass pHs of around 8.2 calcium carbonate will start precipitating and clogging up the aquarium tubes and filters. Inadequate testing when doing water changes with mixed salt-water can cause swings in pH which could lead to this.

Carbon dioxide levels in water will affect reef aquariums pH significantly, as corals will take away CO2 from the water during the daytime hours through photosynthesis, and then generate CO2 when the lights are off. For this reason, a refuge with macroalgae with a lighting cycle inverse to the display tank will help keep pH stable on a reef tank.

Causes for Low pH on Reef Aquariums

Daytime pH under 7.8 is considered a problem, but before you act using a quick fix make sure to check whether any of the following things are causing the problem:

  • Incorrect testing Before taking any sort of corrective measures (besides the mildest ones) check with a different test kit or electronic meter to ensure your pH is indeed below the desired range.
  • Aquarium not fully cycled If your aquarium hasn’t finished cycling the chemistry won’t be mature enough to allow for sustained life.
  • Calcium carbonate/carbon dioxide reactor Adding CO2 to the water will lower the pH, so it may need to be adjusted.
  • Incorrect Aeration There is a reason why you need currents in a reef aquarium, and it’s to ensure the water is correctly aerated. When mixing new water for your aquarium, use a wavemaker or circulation pump on the bucket to ensure the water is properly aerated, as that will balance the CO2 content and increase the pH.
  • Excess CO2 Environment The room may have too much CO2, which in turns makes the tank balance at higher CO2 contents.
  • Carbonate alkalinity is too low You need to look at carbonate alkalinity as a measure of the “buffering capacity” of water, or its ability to resist a change in pH. If it’s too low (for a variety of reasons) the pH will be lower, and it will be difficult to raise it.

How to Increase pH in Marine Aquarium Tanks

In order to increase the pH in reef tanks is better to take a conservative approach. This means doing things that are less drastic first, and evaluating how much improvement you can see. Consider also whether any of the above mentioned problems are the cause for low pH on your aquarium, and correct them accordingly.

General Ways To Raise pH on Reef Tanks

Limewater and some two-part pH additives are commonly used ways to raise the pH on an aquarium without increasing alkalinity (carbonate or calcium related) too much. Buffers are not so useful, as they increase the alkalinity too much which makes it even more difficult to raise the pH further, entering a downwards spiral where need more and more chemicals to balance your aquarium.

Another way to raise pH in a reef aquarium is lowering the CO2 content, and the best way to do this is having a refuge where you can grow macro-algae, and aerating the water better provided the room’s CO2 content is not too high. If your room’s CO2 content is too high, opening a window can help as it will increase ventilation. This problem happens often in winter, when people keep windows closed due to the weather.

Low pH Due To Calcium Carbonate Reactor

A calcium carbonate/carbon dioxide reactor uses acidic carbon dioxide to dissolve calcium carbonate, and in theory that CO2 would be removed from the tank after the process. However, most often than not some acid reaches the tank, lowering the pH. If the reactor is badly adjusted, you can end up with pH too low for a reef tank, so your first port of call should be adjusting the CaCO3/CO2 reactor correctly.

Always ensure the water coming from the reactor is properly aerated in order to reduce the amount of CO2 that actually reaches the tank, however you may need to use a pH increase chemical such as limewater or accept that your aquarium’s pH is never going to be at the high end of things and ensure the remaining conditions (calcium and carbonate alkalinity) are optimal.

When raising the pH on a reef tank always try slower but safer methods such as better aeration, a refuge or adjusting the reactor before you turn to buffers, as some problems can sort themselves out in a couple of days as CO2 is removed from the water, and you could end up with the opposite problem: pH too high on a reef aquarium. And that isn’t fun either!

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