Have you ever been on vacation or at a friend’s house and the drinking water there tasted different than it does at your house? 

That’s because not all water is the same, and this is especially true for aquariums. Every fish and plant species require different aquarium water chemistry.

The water in your tank may or may not be suitable for its inhabitants, and since you’re reading this, I’m willing to bet that you think you’re missing something in your aquarium. 

Keep reading to discover the most important aspects of aquarium water chemistry and what steps you can take to correct issues you may have so that you can bring your aquarium to the next level. 

Look ahead to the topic you want:

• Why is Water Chemistry Important?
• Acidity and Alkalinity (pH)
• General Hardness (GH)
• Carbonate Hardness (KH)
• Nitrates
• Ammonia and Nitrites

What is Aquarium Water Chemistry and Why is it Important?

The vitality of your aquarium and its inhabitants relies almost entirely on the state of your tank’s water chemistry. 

In fact, an unbalanced tank is likely the root cause of any issue you’re currently struggling with or will struggle with in the future.

Poor water chemistry affects the important biological processes of your fish like digestion, respiration, and reproduction, all of which are necessary for a thriving aquarium. 

When things aren’t as they should be, your plants and fish can get stressed, sick, and if allowed to go on long enough, might even die.

I wouldn’t wish that on anyone’s aquarium, and I want to give you the knowledge you need so that you can avoid catastrophe and create an active, healthy environment. 

So let’s dive into the big names in the world of water chemistry.

Aquarium Water Chemistry: Acidity and Alkalinity  

There is MUCH more about the chemistry of pH, but since this is an aquarium blog and not a science class, we’re going to skip over it. All you really need to know is that the pH measures the acidity or alkalinity of the water in your tank. 

pH is expressed on a scale from 0–14, with 7.0 being neutral. If your tank has a pH value of less than 7.0, it’s considered acidic and if it’s greater than 7.0, it’s basic or alkaline.  

In planted tanks, we usually work in a range of 6.4 to 7.4 because that’s where most common species of fish and plants are comfortable living. 

Unless your aquarium inhabitants require very specific pH levels, you should aim to maintain a pH level within the suggested range. 

You can do this by using test strips to monitor pH and using trusted additives to lower or increase pH as needed. These things should be included in your weekly maintenance process. 

Aquarium Water Chemistry: General Hardness

In some areas of the country, the water is extremely hard, meaning that there are a lot of dissolved minerals in it. 

In the hobby, general hardness is often referred to as GH and it can be measured as German degrees of hardness (dGH), parts per million (ppm), or milligrams per liter (mg/L). The last two are the same measurement and are used interchangeably.

GH is the concentration of dissolved minerals in your aquarium water, the two largest components being calcium and magnesium, which is what most test kits measure.

As a general rule, you want your aquarium water to have a GH between 50 ppm and 150 ppm. It’s good for your fish’s health and your plants will thank you because they need a certain amount of calcium and magnesium to grow, and that range is the sweet spot. 

If you tested the water and found that your GH is below 50 ppm, one of the best ways to increase it is to add a small bag of crushed coral to your filter. It will dissolve slowly and release a steady source of hardness and carbonate hardness (discussed in the next section).

Personally, I use Seachem Equilibrium when my GH is low, but that’s only because I think it’s a little easier to control. 

Water being too hard is rarely an issue in a planted tank, but if it’s outside of your comfort level, carry out a water change or purchase products from a trusted aquarium supply store.

Aquarium Water Chemistry: Carbonate Hardness 

Carbonate hardness (KH) is a bit more complicated than general hardness, so I’ll do my best to keep it short and simple. 

KH is the measure of carbonates and bicarbonates in your water. These substances buffer the water and allow you to add CO₂ to your aquarium without letting the pH fall too low. There’s a whole “behind the scenes” relationship between pH, KH, and CO₂, but I’ll explain that in a later article.

You can measure KH with ppm or mg/L like general hardness, or with an old measurement called degrees of carbonate hardness (dKH). People will use either one, so here’s a formula to help you convert between the two:

# of ppm / 17.8 = dKH

The big picture I want you to see is that carbonate hardness plays a key role in maintaining the pH and stability of your aquarium. 

As I said, it acts as a buffer to help counteract changes in pH caused by the addition of tap water, the breakdown of organic materials, and the release of waste from organisms.

You should shoot to have your KH between 3.0 and 8.0 dKH (53.4 ppm and 142.4 ppm). If it’s much higher or lower you’ll experience drastic changes to your tank’s water, which might mean disaster for your setup. 

To increase KH levels, add plain baking soda to your aquarium. ¼ teaspoon will raise the KH of 10 gallons of water by 1 degree (18 ppm).

Be very, VERY careful using commercial buffers to increase KH. A good number of the products out there have phosphate-based compounds which can cause algae problems.

If you want to decrease KH, add peat moss to your tank. It will release tannins and help lower levels over time.

Aquarium Water Chemistry: Nitrates

Nitrates (NO₃^–) are important for two reasons:

1. Once the tank has cycled, rising nitrate levels could signify that you’re overstocking or overfeeding your tank, or not doing water changes frequently enough 

It doesn’t hurt to have moderately high levels of nitrates in your aquarium, but it could be an indicator of waste build-up, which could lead to an algae bloom if you’re not careful. 

2. Nitrates are an important nutrient for plants. 

It’s better to have some nitrates than no nitrates, and your plants will agree. Actually, nitrates are one of the three macronutrients plants need in order to thrive. Don’t let NO₃^— levels reach above 25 ppm for too long and never let it drop down to zero. 

You can increase nitrate levels with fertilizers like Seacehm Nitrogen and you can decrease them by performing a water change.

Aquarium Water Chemistry: Ammonia and Nitrites  

Ammonia (NH₃) and Nitrites (NO₂^—) are normally an issue in the early stages of an aquarium. 

Ammonia is a toxic compound that is produced from waste from fish, uneaten food, and decaying organics. In a new tank, rising ammonia levels can be fatal to fish and other inhabitants, but it’s an important building block during the Nitrogen Cycle.

Nitrites emerge as they break down the toxic ammonia and you’ll see a spike in nitrite levels as this happens. Nitrites are less harmful than ammonia but can cause stress and even death to fish if left unchecked. 

These two can be mitigated by performing water changes as part of your aquarium maintenance two or three times a week in the first month of an aquarium’s life.

But as your tank matures, beneficial bacteria take up residence in your filter, plants absorb nutrients, and the rest of the water components keep things stable. 

As you can see, everything is interconnected. If one thing is off, the whole environment becomes off balance. I would say it’s not a science to figure all this out, but it is. It’s chemistry. But it takes experience and close observation to make sure everything is working in unison. 

And every tank is different right? People have different water depending on where they live, different tank requirements depending on the fish and plants, and different maintenance schedules. 

This article was a broad overview of the most common aspects of aquarium water chemistry and I hope it was helpful to you. 

If you want to learn more about planted tanks and aquariums, take a look at our other blogs. 

Thanks, 

Dom Blank 

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