5-08-2021, 20:06
2 052


The simple answer to the question of when to change the water in the aquarium is when the nitrate level reaches about 40 parts per million, give or take.

That's it, no need to make it more complicated unless you want it to be. Like those of you who may have aquariums where nitrates never reach 40 ppm.

The nitrogen cycle

So, why it's so simple. Well, if you understand the nitrogen cycle, and you know that nitrates are the last step in the cycle and that they are bad for the fish, and that we aquarists should remove them by water changes.

There are many factors that determine nitrate levels and the rate at which they accumulate. This is where the phrase "depending on the circumstances" comes from.

Depending on the size of your tank, the size of your fish and their number, the amount of food, the frequency of feeding, the vacuuming schedule, the efficiency of your filter... You could go on and on. Yes, depending on all this, the rate of increase in nitrate levels will be determined, but the final result will remain the same. When your aquarium's nitrates reach 40 ppm, it's time to change the water.

So, if you reach these levels after a week, change the water weekly. If you get to these values sooner than a week later, and you don't want to do more than one water change a week, consider larger water changes.

But don't just go from a 20% water change to a huge 80% water change. If you decide to make larger water changes, you need to do it gradually to make sure your fish don't react negatively to it.

If you don't reach 40 ppm within a couple of weeks, then change the water every couple of weeks.

We're talking about the 40 ppm value because it's the general consensus in aquaristics, but some larger fish can handle higher concentrations as well.

Water changes are not only for nitrate removal

With all that said, nitrates are not the only reason for water changes.

The aquarium water contains important minerals such as calcium, magnesium and many others, which will eventually be exhausted. Both by the fish and the aquarium itself. And the best way to replace these important minerals is to change the water. Yes, there are products that can add these minerals directly to your aquarium, but they cannot compete with a good big freshwater change.

When to change the water in an aquarium with African cichlids

Nitrates are acidic, and the higher values they reach and the longer they stay in the aquarium, this can deplete the minerals in it that buffer the water and maintain a high and stable pH.

In other words, a high nitrate content may lower the pH value. And those keeping African cichlids know to maintain a high, stable pH for them.

Create a water change schedule for your aquarium

High nitrates disrupt the nitrogen cycle

On the other hand, no matter what kind of fish you keep, if the pH of the water drops too low, you can disrupt the biological balance by killing all the beneficial bacteria, causing an ammonia spike in your aquarium and killing all the fish.

You see, in this beautiful glass water tank where we keep our underwater inhabitants, everything is balanced. It's all cause and effect here. One can easily lead to the other, so you have to maintain stability and balance.

Almost nitrate-free water change

On the other hand, some of us have aquariums that do very well at keeping nitrate levels low.

As in planted aquariums or aquariums with few plants, we would recommend changing the water every 2 to 4 weeks. Although nitrate is not the main problem, replenishing these important minerals, which are essential for the fish and the aquarium, is just as important.


Once you've started your new aquarium and populated it with fish, make a maintenance schedule for yourself, whether it's a weekly water change with a monthly filter cleaning or a monthly water change with two filter cleanings. Whatever your schedule, if it helps to maintain water quality, don't change it! Keep doing what you like and help your fish stay healthy.

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