Lowering Nitrates in a Pond: What Works Quickly, What Works Long-Term, and the False Solutions

The test shows 50, sometimes 100 mg/L of nitrates, the water turns green at the slightest ray of sunlight, and you're looking for the fastest solution. Nitrates are the end product of waste breakdown in the pond: they are only slightly toxic to fish in the short term, but are mainly a fertilizer that feeds algae. There is only one truly quick way to lower them; the others are slower but lasting. Here's how to combine the two.

Where do nitrates come from?

In a pond, almost all the nitrogen comes from food. Fish excrete a large portion of it as ammonia, while uneaten food and sludge also break down into ammonia. The filter's nitrifying bacteria convert it into nitrites, then nitrates. This is an excellent thing, because ammonia and nitrites are much more toxic. But nitrates accumulate: a standard filter does not remove them.

External inputs also contribute: refill water, runoff from a fertilized lawn, fallen leaves, and pollen.

When should you worry?

Nitrates alone don't cause algae: it's the combination of nitrates and phosphates that determines it. Water rich in nitrates but very low in phosphates often remains clear. So measure both.

The quick solution: changing the water

Replacing 20% of the water lowers nitrates by about 20% immediately, provided that the new water contains little nitrate. In Europe, tap water may legally contain up to 50 mg/L of nitrates, and some agricultural regions come close to that level. Test your tap water, or check the analysis published by your water supplier. If it contains a high level, use properly collected rainwater, while monitoring the KH, which it will lower. Carry out 10 to 20% water changes per week rather than one large drain, and treat tap water for chlorine.

Sustainable solutions

Reducing inputs

• Feed according to the temperature and in quantities consumed within a few minutes.
• Choose a quality, more digestible food that produces less waste.
• Adjust the number of fish to the volume and filtration capacity.
• Remove leaves and sludge before they decompose.
• Avoid lawn fertilizer runoff into the pond.

Putting plants to work

Aquatic plants directly absorb nitrates and phosphates to grow. The most effective are fast-growing plants: submerged oxygenating plants such as hornwort, vigorous marginal plants, and floating plants. Remember to regularly remove some of the biomass: each handful removed takes the nutrients with it. A planted filter bed connected to the pond is the most powerful long-term solution.

Supporting the bacterial flora

Maintenance bacteria help mineralize sludge and organic waste before they slowly turn into a nutrient reserve. As for denitrification, which transforms nitrates into gaseous nitrogen, it occurs naturally in oxygen-poor areas, at the heart of the substrate in a planted filter bed or in specific very low-flow filters. A classic, well-oxygenated pond filter has almost no denitrifying effect.

False good solutions

• Zeolite binds ammonium, not nitrates. Useful in an ammonia emergency, useless here.
• Activated carbon retains certain organic molecules, not nitrates.
• Miracle “anti-nitrate” products that promise a spectacular reduction within a few hours without changing the water or reducing inputs: biology does not work that way.
• Complete draining: it stresses the fish, destroys the bacterial balance, and does not address the cause.

Also read

The other half of the equation: Reduce filamentous algae with an anti-phosphate solution

READ THE ARTICLE

Frequently asked questions

— How can nitrate levels be reduced quickly?

Through a partial water change with nitrate-poor water. This is the only immediate method.

— What nitrate level should there be in a fish pond?

Ideally less than 25 mg/L, and as low as possible to limit algae.

— Does tap water contain nitrates?

Yes, sometimes down to the legal limit of 50 mg/L. Measure it before using it for dilution.

— Do bacteria remove nitrates?

Nitrifying bacteria in the filter produce them. Removal through denitrification requires low-oxygen zones, such as the substrate of a constructed wetland.

In summary

• Nitrates are more of an algae fertilizer than a poison.
• Only a water change acts quickly, using nitrate-poor water.
• Less food, less sludge, more plants: the lasting reduction.
• Also measure your refill water.
• Zeolite does not retain nitrates.

Last updated: September 28, 2026.