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Effects of PH in Marine Aquariums

I have learned a great deal about how PH works in our tanks lately. I had a little mishap with my coral quarantine tank and thought I would share some of new new found knowledge with others. While I am by no means a chemist, read on for some simple explanations of why you should monitor your PH!

What is PH?

When we talk about PH, to put it simply, it is the measure of Hydrogen atoms. It can get very complicated with math but we will keep it simple and just state we want to measure the available hydrogen atoms in a specific solution.

Why Measure?

In the reef tank community, we have a general acceptable range of PH from 7.8 up to 8.5. It’s important to check PH every now and then because if it’s out of that range there may be other issues we are dealing with. It’s important to note here that we don’t chase PH numbers. What I mean is if our PH is off, we don’t just add some type of buffer to raise or lower it. We need to use that information to identify the source of the problem and address that instead. PH can be a great tool to diagnose problems in our tank and that is why we measure.

Low PH

Low PH, less than 7.8 at least, can be a problem for our tanks. It can signal several issues that need to be addressed.

Low alkalinity – when we talk about alkalinity, we are discussing the available carbonates in the water for corals to use. We can test this by measuring how much solution in a test kit is required before we hit a PH of around 4. When measured use typically use units in dKH. The ocean, if memory serves me right, have a dKH of around 7-7.5 or so. If you tank dKH is low, your PH can drop as well. We’ll discuss alkalinity in another article in more detail.

Excess carbon dioxide – This one is a common culprit of low PH. Especially seen in newer homes where they are more air tight. First start by placing an air pump in your water and let it sit overnight. Sometimes excess carbon dioxide gets trapped in the water and by putting an air stone in the water we help that CO2 escape.


If the overnight trick didn’t work, you may just have excess amounts inside your home. An easy way to test this is to remove a single cup of tank water and place it outside. Put an air stone in it for about 15 minutes then check the PH again. If it rose by a substantial amount, you likely have excess carbon dioxide in your home. Usually older homes that are drafty don’t face this issue as air sweeps in around the windows and doors.

There are a few ways to fix this. First you can open a window for a while. Easy and handy but not helpful if you live in the North during winter. If you have a skimmer you can extend the air tube to outside a window, getting fresh air injected into your skimmer, or you can run a CO2 scrubber. A third option is the use of a refugium or algae turf scrubber run during the night. Algae of all kinds consume CO2 and produce oxygen during photosynthesis. By running it during the night it can raise PH when it naturally falls and keep it more steady.

Effects of Low PH

Having a low PH can have detrimental effects to your tank. Low PH can mean less oxygen available for your fish and corals. Having too low of a PH can actually cause a process that starts to dissolve calcium in the water. This can lead to calcium being pulled out of the corals in your tank and they start to dissolve.

High PH

Having too high of a PH is not nearly as common as having low PH. Having above 8.5 PH most likely means your alkalinity is very high. The simple solution here is to do a water change to lower your alkalinity. If you are dosing kalkwasser, limewater, or two part dosing you may need to reduce the amount you are dosing. If you don’t want to lower the dKH you can dose something acidic such as vinegar. Vinegar is a carbon source that feeds bacteria on top of being acidic. In turn those bacteria consume oxygen, as well as nitrates, and that lowers the PH. When dosing vinegar be sure to not drop the PH much below 8.5 initially because the bacteria will continue to grow over the next couple days and lower the PH even more. PH will be the highest during the end of the light cycle so if your PH gets too high right at the end you can simply reduce the amount of time your lights are on. You can also add an air pump here to fix high PH. By pushing in air into the tank you are essentially equalizing the carbon dioxide with the room which should keep the PH more level.

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