pH KH & GH explained for the freshwater aquarium

Aug 8

Most aquarium keepers have a rough idea what pH is, but really understanding what it is and how it relates to GH and KH is an important part of keeping a freshwater aquarium. I know this is a topic that a lot of my clients are unsure about, so I figured it would be a topic to write about.

On the surface, it’s fairly simple, but when you take a deeper look it gets complicated. I’m going to start off giving a relatively simple explanation of what pH, GH and KH (the trio) is and how to optimize those levels for your fish. This route you go will be dependent on where you live and what kind of mineral content your tap puts out.

If you’re like me, you might be wondering why certain parameters are important for your fish. Read on to the second part, where I go into more details about the science behind pH mainly and how it affects your livestock. I’ll also include a chart that breaks down some of the more common fish for planted aquariums based on their water parameter preferences.

Don't like reading? Check out the video!

Why’s this important?

The simple explanation of why this is important, is because our critters as well as plants have evolved to live in certain conditions. It’s a little hard for humans to understand how these different water parameters affect fish because our bodies are so different, but for a few example situations - extreme pH levels can burn gills in fish. Lack of minerals will result in shrimp not being able to create their exoskeletons. Low calcium levels can result in poor bone formation in fish.

The trio is something to consider, if you’re getting random die offs of your fish and you’ve tested the big ones like ammonia, nitrites and nitrates. It's definitely a good idea to visit the trio. The trio are often under-looked in fish-keeping and can be fine for a long time, until one day an imbalance results in a pH crash.

Generally, the best approach is to as replicate the natural water parameters that our livestock come from.

pH

Put simply, pH is how acidic or basic a liquid solution is - in this case we’re talking about fish tank water.

It’s measured on a scale of 0-14 of anything under 7 being considered acidic and over being basic. The pH scale is not a linear one, it’s logarithmic. What that means is that every time you jump a digit away from 7 or neutral, you get 10 times more acidic or basic. The further you get away from neutral the magnitude of change increases. A pH of 5 is 10X as acidic as a pH of 6 whereas a pH of 4 is 100 times as acidic as a pH of 6 and a pH of 3 is 1000 times as acidic as a pH of 6. It works the same way with basic solutions. A pH of 10 is 10 times as acidic as a pH of 9 and 100 times as acidic as a pH of 8.

These (picture below) are some common household liquid solutions and they’re varying pHs. One interesting thing to note is that as you get into the extreme pH zones on either end, you start to find corrosive chemicals used for cleaning. It’s interesting for me to look at what water conditions I have and compare it to OJ or bleach for example - good thing my pH isn’t at those levels!

Most aquarium inhabitants need to land somewhere between 6 and 8.5 when it comes to pH.

Chart of various household items and their pH values.

GH

GH stands for the general hardness of water, or the mineral content. It’s determined by the amount of calcium carbonate and magnesium ions present in the water. Most livestock need at least a little bit of these minerals to build bones, teeth or exoskeletons.

Because one standard unit of measurement never seems to be enough, we have three different ways to measure GH. By degrees general hardness represented by DGH, mg/l and PPM.

The scale is measured from 0 - 30 DGH ranging from 0 - 530 PPM with very soft on the lower end to very hard on the higher end. Most of the common livestock in freshwater aquariums prefer the softer end of 4-8 DGH or the medium range of 8-12 DGH.

KH

KH is the carbonate hardness or buffering capacity of aquarium water. It’s important for helping maintain a stable pH in aquariums. A tank with really low KH is susceptible to large, sudden changes in pH which can be stressful at the least for our fishes, and in some cases fatal.

It’s measured the same way that GH is by DKH, mg/l and PPM, and is also broken down into soft or hard brackets like GH. So when you talk about your water being hard or soft, you could be referring to either GH, KH or both.

Generally you want to keep your KH AT LEAST 2 or 3 to avoid any rapid fluctuations in pH. This is what’s known as buffering your water. You’ll still need to double check what parameters your livestock prefer. Also, it’s best to not let your KH go sky high, like beyond 8.

Even though there are general hobby standards, there are always exceptions. Like many Caridina AKA bee shrimp NEED a KH of 0-1, which means I need to take really good care of this tank (picture below) to prevent any pH crashes!

Making adjustments to each element

Adjusting your pH

While there are chemical products you can buy to shift the pH of your aquarium up or down, I do not recommend going that path. I refer to that process as chasing pH and in my experience you’ll end up spending a ton of money on chemicals which only help you achieve your goal for a brief period.

The more sustainable path is to use materials that slowly lower the pH over time as well as focusing on getting your GH and KH right.

Wood and leaves are a terrific way to lower pH a little bit. No matter how many leaves or chunks of wood you use, it won’t drop the pH from a 10 to a 6. They release tannins AKA tannic acids into the water column as they break down. That’s the brown pigment that tints your water. So now you know, when you add wood or leaves to a tank and it starts to turn brown, it’s not that your tank is dirty, just that pigments and tannic acid is being released. You can use peat moss at the bottom of your aquarium and cap it with sand to achieve the same effect.

Rocks such as limestone, aragonite and dolomite will raise the pH in your aquarium.

Adjusting your GH

GH can be raised through two different methods. Unlike with pH, I do approve of using powder products to raise GH. Equilibrium is my go to for raising GH, but there are a ton of other products that you can find out there that’ll do just as good a job. I use this in tanks with really sensitive species that can’t tolerate sudden shifts in GH. That way, I’m adding water with the exact same parameters back in during a water change, rather than waiting for rocks to alter water parameters between water changes.

In most of my tanks with generally hardy species I use limestone to raise the GH. Limestone leaches minerals into the water over time slowly increasing the GH.

I am not a fan of using crushed coral to raise GH because I have concerns over where it’s been sourced from. In some cases coral reefs are destroyed to harvest coral rubble for this purpose, as well as for ornaments.

Unfortunately, lowering your GH is a lot harder than raising it. You’ll need to get a reverse osmosis system or buy water from your fish store. In many cases after water is completely stripped of minerals, people will add a little bit back in, but that will depend on the species of livestock you keep.

Crustaceans, plants and fish will all consume minerals as they grow, but it’s unlikely to have much of an impact on the DGH in your tank unless you are already starting off with next to nothing.

Adjusting your KH

KH in your aquarium can be increased by adding in carbonate or bicarbonate products like baking soda. You can also use limestone like you would to increase GH.

Like GH, you’ll need an RO system to pull all the KH from water.

THE TRIO

So I mentioned the trio earlier, and it’s not just cause I’m lazy and don’t want to keep saying pH, GH and KH. They all go hand in hand in the aquarium.

Generally as the GH and KH rises, we see the pH go up. Basically the way that works is that the stones like limestone, crushed coral, dolomite and aragonite release calcium carbonate (calcium for GH and carbonate for KH) into water raising both of them at the same time.

However, it’s not the calcium that’s impacting the pH, its the carbonate. Carbonate neutralizes acid, or it buffers it. That’s why higher KH or carbonate is important for preventing pH crashes.

But because we usually find the minerals associated with KH in the same natural sources that we find those associated with GH so we tend to think of all the elements of the trio as being related. As you raise one, the others tend to rise.

It is possible to buy products that will only raise one or the other. For example Seachem Equilibrium raises GH without altering KH. Sodium bicarbonate, or baking soda would raise the KH of a tank without impacting the GH. This alkaline buffer raises the KH and pH of the aquarium.

The Deep Dive

Alright, let's take a little deeper dive and talk about what all of these mean, and how it impacts our livestock. I’m not going to dive into as much detail as you’d get with a chemistry class, but I am going to get into the science behind these topics in a little more detail, so if you find that boring, now's a good time to bow out.

pH

Now that you know a bit about the pH scale and how that works, let me explain more about the chemicals responsible for everything and how that relates to our livestock.

pH stands for power of hydrogen. The more acidic a solution or water, the higher the hydrogen ion concentration. Hydroxide ions work the same way, except for basic solutions.

Ions are atoms or molecules that have lost or gained an electron giving them a charge - that’s the ion part, being charged. Hydrogen atoms lose an electron and gain a positive charge. The resulting ions want to bond with minerals. This bonding process can result in degradation of the materials which they are interacting with. Because of the attraction to minerals, acids are used to brighten up metals and clean oxidized surfaces. Like if you had mineral deposits on the glass of your fish tank you’d use hydrogen peroxide or vinegar to help loosen it up.

Hydroxide ions gain an electron resulting in a negative charge. These tend to be attracted more to oils, fats and proteins. The more basic an element, the greater its ability to denature oils and fats. Bleach for example with a pH of 13 is terrific at cleaning up greasy stuff.

When you think about it in terms of an aquarium, a really high pH like 9 or 10 will start to denature slime, mucus coats and even exterior cell membranes of aquatic critters.

Another negative to high pH is that ammonium is converted into ammonia which is very toxic to fish. In most cases, you shouldn’t have ammonium in your cycled aquarium anyways so it shouldn’t really matter, but if you do get a little spike, the impact will be far more profound at a higher pH.

With low pH on the other hand, we see that nitrites are far more toxic. If you are trying to cycle an aquarium in a low pH environment it will take a lot longer and the nitrite phase will be deadly, which just means you’ll have to wait longer to put fish in your tank. That’s why it’s important to not overdo it with CO2 while cycling an aquarium. I’ve thought before that it didn’t really matter since there were no fish in the tank, but it does impact beneficial bacteria growth rate.

Outside of the cycling phase low pH is still dangerous for fish. It basically burns them, like a strong acid can give a person an acid burn!

Acceptable parameters

Even though it’s generally best to try and replicate a fish’s natural water parameters, it’s not always necessary. Truthfully, a lot of fish have the ability to live healthy lives outside of their natural pH range. If you’ve made it this far in the blog, I’m going to assume you’ve watched a fair bit of fish YouTubes. Think about people like MD FishTanks, Father Fish or Ohio Fish Rescue that are mixing all sorts of fishes from different places. Their fish seem to all be living their best lives.

The general rule of thumb is that most fish can live healthy lives between 6 and 7.5 even, tolerating ranges of 4.5 to 9.5 for brief periods, not that I’d advise either end of that scale. And even though we like to keep our aquariums as stable as possible, it’s pretty normal for fish tanks to see pH swings from the 6 to 7.5 range over the course of a day. Keep in mind that the pH scale is logarithmic, so changes around the neutral zone aren’t as drastic as changes from 6 to 5 or 4.5 for example.

This cyclical fluctuation is particularly common in planted tanks. During the day, plants consume CO2, which is acidic and produce oxygen, which is basic. In the evenings, when lights are off plants respire, consuming oxygen and producing CO2, resulting in acidification of a tank.

Even though a lot of fish are OK with this kind of cycle, there are some that are not. It’s important to do your research before getting fish, to know how sensitive they are to changes.

If you use CO2 in your tank, then you’ll notice that the pH of your tank shifts when it’s saturated vs having been off all night. When CO2 enters the water, it reacts with it becoming carbonic acid, which is why you’ll see a pH shift when using carbon dioxide.

Something I’ve heard discussed on more than one occasion is that pH swings due to CO2 are not harmful for fish, it's only pH swings related to low mineral content that are bad for our fish. In that same frame of mind, I’ve read some articles about people who drop their pH to 5 when CO2 is fully saturated. Personally I would not go there. I try to keep my range between 6 and 7.5.

As far as pH swings due to low mineral content, it’s true, that happens. If you’ll remember about the buffering capacity of minerals like carbonate, because it wants to bond with those hydrogen ions with a positive charge, thus neutralizing acid in the water. A tanking pH in the absence of minerals usually spells problems for our fish. And it’s not necessarily because the pH is too low, though it might be. Low pH is a symptom of a lack of minerals which are important for the fish and shrimp to function.

Calcium and magnesium -> the elements that comprise general hardness are also important for internal functions of fish. I already talked about bone and scale formation but these minerals are also important for regulating gill functions and changing osmotic pressure.

Osmotic pressure is basically referring to a little levy system inside fish. Sometimes they need to change their permeability to water based on the changes in mineral or salt concentrations around them. Think about how wrinkly our skin gets when we take a bath. That’s cause our bodies are not as efficient in managing different water concentrations in our body versus what’s around us. Since fish spend their whole lives in water, they’re masters at it, but they require the right minerals (magnesium and calcium) to do that.

Plants

Alright, let's talk a little bit about plants. I’ve got a lot less to say about plants. I don’t feel bad experimenting with them I guess. If some don’t make it, not a big deal. Most plants will do fairly well though if you give them some light and soil, maybe some liquid fertilizer. Add in CO2 and you can keep most any aquatic plant in your aquarium.

But for those really sensitive or difficult plants… Why do they need soft and slightly acidic water?

Hard water, with high mineral content is bad for plants. Even though plants require some magnesium and calcium, in really high concentrations, those ions will basically block absorption of other nutrients. Over the long run, those minerals can also clog up the soil, making water, gas and nutrient exchange less efficient around roots.

Really hard water can also gunk up plant pores, leading to poor gas and nutrient exchange within leaves.

The reason that low pH is beneficial for aquarium plants, as well as terrestrial plants is that it allows nutrients to dissolve better before being taken in by plants. That’s the case for most micro and macro nutrients, but a few like magnesium, calcium and molybdenum are absorbed better at slightly higher pHs.

Again, most plants are pretty hardy as long as you give them nutrients and light. So feel free to experiment with what does well in your tank though some trial and error. Even in my high tech tanks, some plants just don’t seem to do well, while 95% thrive.

That’s it!

I hope you’re feeling riveted after reading up on pH, GH and KH. You might be a little bit bored if you’ve made it this far, but you’re that much more knowledgable now. Drop a comment if you have any questions!

Michael Amato https://freshflowaquatics.com