Lowering nitrates in an aquarium: find the source, act quickly, and stabilize it for the long term

The test has turned dark red. Nitrates are well above your target, and the temptation to empty half the aquarium immediately is strong. That is not necessarily the best idea. To reduce aquarium nitrates sustainably, you first need to know where they come from: tap water, excess food, an overcrowded tank or a clogged filter. Only then should you combine the different approaches, from the quickest to the most sustainable, while avoiding sudden changes that stress fish more than the nitrates themselves.

Where do nitrates come from?

Nitrates are the final product of the nitrogen cycle in an aquarium. Fish release ammonia, which filter bacteria convert into nitrites, then into nitrates. The latter are much less toxic than the first two, but they accumulate: nothing in a conventional filter removes them. Only water changes, plants and a few specific processes reduce them. If you have questions about the earlier stages of the cycle, our article What are nitrites in an aquarium? explains them in detail.

A high level is therefore not abnormal in itself: it indicates that production exceeds removal. To take effective action, you need to know which side of the balance the problem lies on.

Step 1: test the tap water

This is the step most often overlooked, yet it determines everything that follows. Tap water may contain nitrates, particularly in agricultural regions. European regulations set a limit value of 50 mg/l for drinking water: perfectly compliant water for consumption can therefore add significant amounts of nitrates to an aquarium.

Test your replacement water using the same test as for the aquarium. Three situations can arise:

• Low-nitrate tap water: water changes will be very effective.
• Moderate-nitrate tap water: water changes help, but the level will never drop below that of the tap water.
• High-nitrate tap water: you will need to treat the replacement water or use nitrate-removing filter media.

Step 2: well-calculated water changes

A partial water change removes a fraction of the nitrates proportional to the volume changed, replacing it with tap water and its own nitrates. The calculation is simple, and it prevents many disappointments. The table below illustrates a common case: an aquarium at 80 mg/l, tap water at 10 mg/l, without taking into account the nitrates produced between two changes.

Several closely spaced partial water changes produce a result close to that of one large change, while avoiding abrupt fluctuations in temperature, pH, and hardness. This is especially important if nitrate levels have risen slowly over several months: the fish have adapted to them, and a massive water change may shock them more than the nitrate level itself.

With every water change, siphon the substrate: a large part of the waste that fuels nitrate production is found between the grains. For instructions, see our article How to change an aquarium's water.

Step 3: reduce production

Changing the water without reducing inputs is like emptying a bathtub while the tap is still running. The main sources of nitrates are well known:

• Food: every pellet you feed eventually becomes nitrogen in the water, in one way or another. Feed small amounts that are eaten within a few minutes, and schedule one fasting day per week for most adult fish.
• Population: an overcrowded tank produces more than maintenance can compensate for. If nitrate levels keep rising very quickly, the population may simply be too large for the tank's volume.
• The filter and prefilter: sludge accumulated in a prefilter sponge continues to decompose. Rinse mechanical sponges regularly in aquarium water, without cleaning all the biological media at the same time.
• Organic debris: dead leaves, a dead fish forgotten behind the décor, trapped food. A quick inspection prevents many surprises.

Step 4: put plants to work

Aquatic plants take up nitrogen for their growth, and nitrates are among their preferred sources. Not all plants are equal: slow-growing plants consume little, while fast-growing and floating plants consume much more. In a moderately stocked aquarium, good plant cover can keep nitrates stable at a low level over the long term.

A few principles:

• favor fast-growing stems and a few floating plants, whose dangling roots draw nutrients directly from the water;
• prune regularly: it is by removing biomass that nitrogen is actually removed from the system;
• make sure they get sufficient light to grow, without excess that would encourage algae.

Step 5: filter media, when and why

There are two categories of solutions for going further, especially when tap water contains high nitrate levels.

Resins and media that remove nitrates

Placed in the filter, they remove nitrates from the water passing through them. They act fairly quickly and are useful for correcting a situation or treating nitrate-laden replacement water. Their capacity is limited: they must be replaced or regenerated according to the manufacturer's instructions, and the level must be monitored to know when they are saturated.

Biological denitrification media

Other media promote the development of bacteria in the filter that can convert nitrates into gaseous nitrogen. Their action is slower but more sustainable. They are well suited to heavily stocked aquariums or tanks where you want to space out water changes, but never eliminate them.

In both cases, these products are supplements. They do not compensate for overstocking or overfeeding.

What are the real risks to fish?

Nitrates are significantly less toxic than ammonia and nitrites, which is precisely why we sometimes let them rise without realizing it. The effects of a high level develop slowly: less colorful fish, slower growth, weakened defenses against parasites and bacteria, and more difficult breeding. Species originating from very nutrient-poor waters, such as many Amazonian fish, and invertebrates such as shrimp are generally more sensitive to them than goldfish or hardy livebearers.

A high level is also an indicator: it signals that the water is loaded with waste, which often goes hand in hand with high phosphate levels and an algae bloom. Lowering nitrates therefore often solves several problems at once. Conversely, a zero level in a heavily planted aquarium can slow plant growth: the goal is not zero, but a moderate and stable level.

Finally, keep in mind that a fish that is unwell does not necessarily have a nitrate problem. Before attributing everything to this parameter, also check nitrites, pH, temperature, and oxygenation.

Mistakes that prevent nitrate levels from dropping

• Never testing tap water: you keep carrying out water changes that cannot lower the level below a certain threshold.
• Cleaning the entire filter with tap water: this destroys beneficial bacteria and risks causing an ammonia and nitrite spike, which is far more dangerous than nitrates.
• Changing a large amount of water all at once after months without maintenance: the shock can be fatal to weakened fish.
• Adding fish when nitrate levels are already high: production increases, and the problem worsens.

A typical two-week plan

For a community aquarium with excessively high nitrates, here is a realistic sequence:

1. Day 1: test the aquarium and tap water, perform a 25% water change with siphoning, rinse the pre-filter, and reduce feeding.
2. Day 3 or 4: perform another 25% water change, inspect the decor, and remove debris.
3. Day 7: test, perform a third 25% water change if necessary, and add fast-growing plants.
4. Day 14: test; if the level rises quickly between water changes, consider adding filter media or reducing the population.

Next, a weekly water change of around 20 to 30% is enough in most tanks to keep the level stable. To choose your testing tools, also see our selection of freshwater aquarium water tests.

Read also

To understand where nitrates come from and their role in the tank: What are nitrates in an aquarium?.

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Frequently asked questions

— What nitrate level should you aim for in a freshwater aquarium?

As a general guideline, many aquarists aim for less than 25 to 50 mg/l in a community aquarium, and lower levels for sensitive species such as some shrimp or very soft-water fish. The key is to avoid a continuous increase from one week to the next.

— Why do my nitrates remain high despite water changes?

Most often, tap water already contains nitrates, or the aquarium produces more than water changes remove: too much food, a dense population, a clogged filter, or soil that is never siphoned.

— Can you change all the water to lower nitrates?

It is not recommended. A massive water change abruptly alters the temperature, pH, and hardness, stressing the fish more than a high nitrate level would. Several closely spaced partial changes are safer.

— Are plants enough to remove nitrates?

In a well-planted, moderately stocked aquarium, fast-growing plants consume a significant proportion of the nitrates. In a heavily stocked tank, they help but are generally not sufficient on their own.

In summary

• Measure the nitrates in BOTH the aquarium AND the tap water before taking action.
• Prefer several closely spaced partial water changes over one very large change.
• Reduce feeding, siphon the substrate, and maintain the pre-filter.
• Add fast-growing plants and floating plants.
• If the tap water contains high nitrate levels, anti-nitrate filter media or suitable replacement water becomes necessary.
• Aim for a gradual decrease and a stable level over time.

Last updated: October 3, 2026.