How to Lower Nitrates in Fish Tank

Having high nitrate levels in your aquarium can be extremely harmful to your fish. Nitrates are a byproduct of the nitrogen cycle and naturally accumulate over time in tanks. While low levels are safe, anything over 20 ppm can start impacting fish health. Luckily, there are many methods available to help reduce and control nitrates.

What Are Nitrates and How Do They Accumulate in Your Tank?

Nitrates, chemical compounds containing nitrogen and oxygen, are a natural byproduct of the nitrogen cycle in aquariums. This cycle involves the breakdown of fish waste and uneaten food into toxic ammonia by bacteria. Next, different bacteria convert the ammonia into nitrite. Finally, beneficial bacteria transform the nitrites into less harmful nitrates.

While nitrates are significantly less toxic than ammonia and nitrite, they can still build up to dangerous levels if left unchecked. Nitrates accumulate in the tank water over time as fish produce waste. The primary way nitrates are removed from the system is through water changes. Without regular water changes, nitrates cannot exit the tank and thus continue increasing.

Factors that can cause nitrates to spike faster include overfeeding, overstocking, and inadequate filtration. Addressing these underlying issues is key to controlling nitrate creep. Still, even well-maintained tanks will see rising nitrate levels between water changes. That’s why additional nitrate management strategies are essential.

What Are the Potential Effects of High Nitrates on Fish?

Prolonged exposure to nitrates over 20 ppm can negatively impact freshwater and saltwater fish health in various ways:

  • Increased susceptibility to disease
  • Osmoregulatory stress
  • Reduced growth rates
  • Lowered oxygen carrying capacity of blood cells
  • Decreased appetite
  • Lethargy and respiratory issues
  • Poor coloration

High nitrates have also been linked to fish fry mortality and adverse effects on specific species like clown loaches.

Furthermore, elevated nitrates can promote algae growth in the tank, fueling nuisance algae and cyano bacteria blooms. This deteriorates water quality and aesthetics.

For all these reasons, keeping nitrates within safe parameters is crucial. But what exactly constitutes a safe level?

Target Nitrate Levels for Freshwater and Saltwater Aquariums

Nitrate levels below 20 ppm for freshwater tanks are generally considered safe for most community fish. Still, sensitive species may show signs of stress above 10 ppm.

With saltwater tanks, target levels should be lower due to the increased sensitivity of most marine fish and invertebrates. Aim to keep nitrates under 10 ppm, with levels closer to 5 ppm being ideal.

These targets can vary slightly based on individual tank conditions and inhabitants. For example, fish-only freshwater systems can often tolerate nitrates up to 40 ppm without issues. Know your livestock and monitor their condition at different nitrate concentrations.

The key is keeping levels stable and within acceptable limits for your system. Nitrate spikes are more dangerous than stable elevated nitrates.

Natural Methods to Lower Nitrates in Your Aquarium

There are several natural and non-chemical options for reducing nitrates in your tank. While not as rapid-acting as chemical media, these methods gently decrease nitrates without drastic swings slowly.

1. Live Aquarium Plants

Live plants are a popular natural choice for controlling nitrates. During photosynthesis, plant roots absorb and metabolize nitrate as a nutrient source. Fast-growing stem plants like hornwort, anacharis, and floating plants like duckweed or water lettuce are especially efficient options.

The more plants you have and the faster their growth rate, the higher the nitrate reduction. Just be mindful of plant lighting and fertilization requirements to sustain this growth. Introduce plants gradually to prevent rapid nitrate drops that could shock fish.

2. Clean Up Crew Additions

Certain algae eating fish like plecos and otocinclus catfish can graze constantly and produce significant waste to fuel the nitrogen cycle. Adding a clean up crew of these fish sparks nitrate production.

However, their waste also provides nourishment for the tank’s nitrifying bacteria. The boosted bacterial colonies drive the system’s biological filtration and nitrate processing power. This greater bio-capacity can gradually lower nitrates.

3. Improved Mechanical Filtration

Optimizing mechanical filtration helps remove organic debris before breaking it into nitrate-producing ammonia and nitrite.

Frequently rinsing filter media in old tank water to clear trapped gunk can improve mechanical filtration efficiency. Installing a micron filter pad and increasing filter cleaning intervals are other options. Reducing debris translates to lower nitrate generation.

4. Target Feeding

Carefully feeding your fish only what they need and targeting feeding to species that need it most can reduce waste production. Use sinking wafers for bottom dwellers rather than flake foods that disperse through the whole tank.

Observe fish activity and appetite when fine tuning feeding amounts. Uneaten food quickly contributes to rising nitrates. Target feeding allows cutting back while still properly nourishing fish.

5. Deep Sand Beds

Deep sand beds in saltwater reef tanks develop complex ecosystems of anaerobic bacteria deep within the substrate. These bacteria actively process nitrate as an alternate oxygen source, converting it into nitrogen gas.

Sand beds 6 inches or deeper efficiently eliminate nitrate through this process. Shallow layers lack the right anaerobic conditions. Limit disturbance of deep beds to maximize denitrification.

6. Refugiums

Refugiums are a powerful natural tool for saltwater tanks. These separate chambers are connected to the main tank and house macroalgae like chaetomorpha. The algae rapidly uptake nitrate for growth while also producing oxygen.

Refugium lighting is tailored to foster algae growth. Harvesting excess algae then directly removes significant amounts of nitrate. These units also cultivate populations of copepods and amphipods that serve as natural food sources.

7. Partial Water Changes

Partial water changes remove some accumulation of nitrates from the water column. This is especially true if using water with very low nitrates. For freshwater tanks, 25-50% weekly water changes are recommended. Saltwater tanks benefit from 10% bi-weekly changes.

When possible, gradual water changes avoid sudden parameter shifts. Only change enough water to get nitrates within target levels for your system. Combine with other methods for a well-rounded approach.

Using Chemical Media to Reduce Nitrates

Chemical filter media that absorb and eliminate nitrate offer a more immediate solution. These media rapidly lower nitrate concentrations but require more frequent replacements. They also create fluctuations compared to slow-acting natural methods.

Here are some of the most common chemical media options:

Nitrate Absorbing Resins

Ion exchange resins like Seachem’s Matrix or API’s Nitra-Zorb work by trapping nitrate molecules in microscopic pores throughout the spherical beads. Flow exposure maximizes contact with the resin. Nitrate is permanently removed from the water until the resin fully saturates.

These media are highly effective at bringing down nitrate levels quickly. However, they have limited absorption capacity and require frequent regeneration or replacement to avoid nitrate leaching back into the tank.

Denitrating Filter Media

Media like SeaChem DeNitrate or Brightwell Aquatics’s MicroBacter7 utilize heterotrophic bacteria strains that break down and consume nitrate as an energy source. The bacteria are immobilized on a solid porous structure with a large surface area for colonization.

The bacteria colony digests nitrate continuously, providing sustained reduction. However, bacteria populations need maintenance and proper conditions to thrive. Media may need replacement every 6-12 months as bacteria lose effectiveness.

Carbon Dosing

Carbon dosing techniques like vodka or sugar in saltwater reef tanks can lower nitrates. The addition of simple organic carbon compounds fuels a bacteria bloom. Rapidly multiplying bacteria use up nitrates and phosphates. Once carbon sources are depleted, bacterial populations crash along with nitrate levels.

This inexpensive but more involved method requires close monitoring and careful carbon dosing. Imbalanced dosing can cause more harm than good for coral tanks. Only attempt under guidance from experienced reef keepers.

Chemical Media Drawbacks

While chemical media provide a quick fix, take care to avoid over reliance on them. Use sparingly and in combination with natural methods for a balanced approach. Drawbacks include:

  • Expense of frequent media replacements
  • Potential for rapid parameter shifts
  • Temporary solution that does not address root causes
  • May disrupt balance of natural biological filtration

Moderation is key when using chemical nitrate removers in your tank. Still, they can serve as a helpful tool when used properly.

Recognizing Signs of High Nitrate Levels

Keep an eye out for these common indicators that your tank may be experiencing elevated nitrates:

Excessive Algae Growth

Rapid spread of green hair algae and brown diatoms often signals an abundance of nitrates. Nuisance algae like these thrive off the excess nutrients.

Strange Livestock Behavior

Fish that are lethargic, gasping at the surface, or showing signs of stress can point to water quality issues like high nitrates. Know your inhabitants’ normal behavior to spot problems.

Poor Growth Rates

Lackluster fish growth, especially in juveniles, could stem from nitrate inhibition. This may be evidenced by small size for their age or prolonged time to reach maturity.

Loss of Appetite

A drop in appetite and lack of interest in food among tank residents may be tied to high nitrates making fish feel unwell. Always rule out water parameters when dealing with appetite changes.

Increased Disease Frequency

Exposure to elevated nitrates wears down fish immune systems over time. This shows up as more frequent disease infections, like ich outbreaks. Improving water quality helps prevent this.

Nitrate Test Results

Testing is the only way to confirm your tank’s nitrate status truly. Simple liquid test kits for nitrate are widely available. Test regularly to detect rising levels before fish show symptoms.

Aim to keep nitrates at the following levels, based on tank type:

  • Freshwater – Under 20 ppm, ideally closer to 10 ppm
  • Saltwater – Under 10 ppm, with under 5 ppm being optimal

Maintaining Proper Nitrate Levels Long-Term

Achieving temporary nitrate reduction is only half the battle. The key is keeping nitrates consistently low through ongoing management strategies. Here are some tips:

Consistent Testing

Test nitrate levels weekly or bi-weekly to remain stable within target ranges and catch any spikes early. More frequent testing after changes helps assess impact.

Strategic Water Changes

Water changes are vital for diluting nitrates. For freshwater, aim for 25-50% weekly. Saltwater benefits from smaller 10% changes every 2 weeks. Match change amounts to nitrate accumulation rates in your tank.

Address Overfeeding

Overfeeding is a prime contributor to nitrate spikes. Feed no more than what your fish can consume in a few minutes 1-2 times daily. Avoid flake foods that quickly deteriorate.

Live Plants and Algae

Sustain balanced plant and algae growth through proper lighting and nutrients. This biological filtration helps process nitrogen waste naturally. Prune and thin plants when overgrown.

Filter Cleaning and Maintenance

Keep filter media clean through monthly rinses and regular replacements. Clogged media causes ammonia spikes that fuel more nitrates. Follow manufacturer advice on media change frequency.

Target Stocking Levels and Species Selection

Stock conservatively, especially in small tanks. Aggressive or very messy fish increase bioload. Research species waste output and stock accordingly.

Quarantine New Additions

Quarantining new fish prevents introduction of pathogens that could spread and cause additional losses. Fewer fish means lower nitrates.

Use Chemical Media Sparingly

Limit use of chemical media only when needed to avoid parameter shifts. Rely more on sustainable natural methods for day-to-day control.

Special Considerations for Managing Nitrates in Saltwater Tanks

Marine fish and invertebrates are generally much more sensitive to nitrate accumulation than their freshwater counterparts. Even small amounts can negatively impact health. Here are tips tailored for reef tanks:

Target Levels Under 5 ppm

For mixed reef tanks, target keeping nitrates consistently below 5 ppm, with under 1 ppm being ideal. Even 10 ppm can be problematic in sensitive reef systems.


Refugiums with macroalgae are extremely effective nitrate sponges for saltwater tanks. Aim for at least 10% of the display tank volume. Prune algae regularly to export nitrates.

Live Rock and Deep Sand Beds

Quality live rock and deep sand beds (at least 4-6 inches) harbor bacteria that metabolize nitrate through anaerobic digestion and denitrification.

Skimmers and Carbon Dosing

Protein skimmers actively strip waste from the water column before breaking it into nitrate. When needed, careful carbon dosing can accelerate denitrification.

10% Weekly Water Changes

Saltwater tanks require more frequent but smaller water changes than freshwater setups. Shoot for 10% weekly to clear nitrates before they accumulate.

Reduced Feeding Frequency

Only feed 1-2 times weekly for reef species like corals or anemones. Herbivores like tangs may need more frequent vegetable matter. Remove uneaten food promptly.

Nitrate Reducing Media as a Last Resort

Avoid nitrate absorbing resins or chemical media unless dealing with a serious algae outbreak. Use temporarily and sparingly to prevent pH crashes in sensitive reef environments.

Frequently Asked Questions About Nitrates

1. How can I lower nitrates without water changes?

The most effective methods for reducing nitrates without water changes include optimizing biological filtration through live plants, algae, deep sand beds, quality live rock, and clean up crews. Target feeding and mechanical filtration improvements also help lower nitrate generation.

2. What fish are most sensitive to nitrates?

Species particularly intolerant of nitrates include wild discus, neon tetras, German blue rams, cardinal tetras, and many dwarf cichlids. Saltwater fish often affected include mandarin dragonets, seahorses, and anthias. Know the nitrate tolerance of your stock.

3. Can nitrates cause fish to gasp at the surface?

Yes, one symptom of prolonged exposure to high nitrates is labored breathing and gasping at the surface as fish become oxygen deficient. The reduced blood oxygen carrying capacity impairs respiration. Promptly addressing high nitrates can help resolve this.

4. How long does it take for nitrates to go down after a water change?

For freshwater tanks, nitrates typically take 12-24 hours to creep back up after a large water change slowly. Saltwater tanks may see increases within 12 hours due to higher stocking levels. The more water volume changed, the longer reductions last.

5. Will plants reduce nitrates in a fish pond?

Absolutely. The same floating and emergent pond plant species that work in aquariums can significantly absorb nitrates in outdoor ponds. Aim for at least 20% surface coverage with faster growing varieties for best effect.


Controlling nitrate accumulation is essential to maintaining any healthy aquarium or pond. While low levels under 20 ppm are generally safe for freshwater systems, saltwater tanks require extra vigilance to keep nitrates below 5 ppm.

Leveraging natural methods like live plants, algae, clean up crews, and target feeding helps control nitrate levels gently over the long-term. Chemical media and water changes can also play a role when used judiciously.

Test frequently, know your livestock’s needs, and react quickly to spikes. Consistency is key for stable nitrate levels. A multi-pronged approach focused on reduction and prevention helps create clear, algae-free water with thriving inhabitants.