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How to maintain pH in an Aquarium

One of the keys to succeeding with an aquarium is providing stable, healthy water parameters for your fish. Maintaining pH where your fish can thrive is a part of this equation. However, it is often confusing to the average aquarist on how to achieve the proper pH. In this blog, I will try to break this down and explain how to best keep pH in the proper range for you and your fish.

What is pH?

The term “pH” refers to the concentration of hydrogen ions in a liquid, or the “potential of hydrogen.” The pH value assigned to a liquid is how we scientifically categorize it as either “acidic” or “basic.” pH exists on a scale from 0-14, with 0 being most acidic and 14 being most basic. A pH of 7 is considered “neutral” and is often where most freshwater fish thrive (anywhere from 7.0 to 7.8 is ideal).

Over time, the pH in your tank will naturally tend to drop. Especially if the KH, or carbonate hardness, is low. Having a KH of 3 or greater will help keep your tank from “crashing” or dropping dangerously in pH and causing a potentially lethal ammonia spike. This level of carbonate hardness is what’s known as a tank’s “buffering capacity”, or the degree to which it can withstand the acids or waste produced by your fish over time. Tanks with a higher KH are inherently more stable, although not ideal for all fish (as this tends to correlate with a high pH).

Knowing your Source Water

Before doing anything to alter your water, you’ll want to test your tap water or source water pH, KH, and GH. You’ll also want to test your tank once it has been set up and running for a few days prior to adding any fish. Things like driftwood or seashells can impact these parameters. Generally speaking, what KH, GH, and pH values come out of your tap water or source water are going to give you the best idea of what you’re dealing with and how to proceed. It’s always best to try to get fish that go with your tap water rather than the other way around, but with enough dedication, you can tweak your water to meet the needs of whatever fish you desire. 

Knowing your Fish

It’s important to research the needs of fish before you set out to keep them in aquariums. While a generally neutral to slightly basic pH is tolerated by most aquarium fish, there are some groups of fish that will not thrive unless their specific pH needs are considered. It’s also an important consideration when you are trying to successfully breed and raise certain fish. For certain species of dwarf cichlids, for instance, their eggs won’t hatch unless the pH is below 6.0. Other fish, like African rift lake cichlids, require higher pH to thrive and reproduce. It’s worth noting that wild caught fish are far more sensitive to pH than tank raised strains. Whatever pH your fish requires, you’ll want to keep it as stable as possible to avoid undue stress. Outlined below are some strategies for lowering or raising pH. 

Lowering pH- Cutting with RO

Though there are products on the market to quickly lower pH, the best way to maintain a lower pH is to cut your tap water with RO water 50/50 and stick to this recipe with every water change (making sure to do your water changes frequently). By doing so, you are diluting the alkalinity of your tap water and therefore lowering the pH. You can either get an RO system hooked up for this purpose or just buy RO or distilled water from the store by the gallon as needed.

Adding tannins

If you have naturally hard water and want to lower the pH slightly, or if RO water by itself is not acidic enough, adding tannins or blackwater extract may help. A lot of fish appreciate the extra cover provided by things such as driftwood and leaf litter, which will slowly release tannins into the aquarium. Tannins will naturally tint the water a yellowish color. If this bothers you, you can combat these tannins with Seachem Purigen while still maintaining the pH-lowering benefits of leaf litter and driftwood.

Raising pH

Depending on where you live, you may have the problem of tap water that is too soft, or low pH. One way you can combat this, especially if you are keeping marine fish or African Cichlids, is to add a crushed coral substrate or a bag of crushed coral to your filter. This will naturally add minerals to your water and raise pH slightly. You will likely also want to use some sort of buffer if this doesn’t boost it enough. It’s best to figure out what ratio of buffers get your water to the pH you want before getting any fish. You can test these parameters with these products:

The API pH and high range pH test kit in conjunction with a GH/KH test kit will cover all your bases when it comes to tracking your pH. The Fluval pH test kit is advantageous when dealing with pH lower than 6.0

Reconstituting RO/Adding Buffers

If you have an RO system set up and are able to collect enough water for your needs, you have another option to maintain pH: You can simply add buffers in the right ratio to target exactly the pH you want to have. This is, of course, a lot more work than just using your tap water and getting fish that can adapt to it, but in the case of some prized display fish (such as Discus), this can pay off. We use and recommend Seachem Alkaline Buffer with Seachem Acid Buffer and Seachem Equilibrium


Once you know your source water and what fish you would like to keep, you can make the decision on how to treat your water going forward. This should ideally be the first step before embarking on your aquarium journey. Alternatively, if you’ve been keeping hardy species and not really looked into pH, but would like to try your hand at more challenging species or perhaps breeding them, then now may be the time to start testing and adjusting your parameters. Almost any aquarium will benefit from keeping tabs on KH at the very least to avoid a pH crash, though consistent water changes will often mitigate this. Getting in the habit of testing your water consistently (weekly, bi-weekly, or monthly) is crucial to maintaining pH and keeping tabs on your aquarium. 

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