How to Reduce Nitrates in a Saltwater Aquarium

Having a healthy saltwater tank requires weekly maintanence.
Having a healthy saltwater tank requires weekly maintanence. (Image: blue damsel image by Tabitha Little from

Nitrates accumulate in a tank as a result of animal waste building up over time. Ammonia is released from fish waste and from uneaten food as it rots. Ammonia is extremely toxic, but in a healthy tank it is quickly converted to nitrites by nitrosomonas bacteria and then to nitrates by another bacteria, nitrobacter. Anaerobic, denitrifying bacteria can live only in marine tanks that have deep, live-sand substrates creating anaerobic pockets where oxygen doesn't reach. These anaerobic bacteria can break nitrates down into harmless nitrogen gas. Even with this kind of deep, live-sand aquarium setup, it is still necessary to do water changes regularly.

Things You'll Need

  • Bucket
  • Sea salt for marine aquarium use
  • pH meter
  • Marine water-testing kit
  • pH-adjusting chemicals
  • DI or RO water
  • Gravel siphon
  • Macro algae
  • Refugium

Change 20 percent of the water to control the nitrates in a marine tank using aged salt water. Mix the salt water up in advance in a bucket using aquarium-approved sea salt. If your nitrate levels are very high, change the water every two or three days until they come down.

Test the pH as well as the nitrates. If your pH is low, slowly adjust the pH higher with aquarium-safe chemicals between the repeated water changes.

Use RO (reverse osmosis) or DI (deionized) water in the fish tank. Tap water contains nitrates.

Feed the fish and invertebrates just the right amount of food. Do not overfeed, as the excess food increases the nitrates as it breaks down.

Vacuum the gravel with a gravel siphon as you do the water change to remove mulm and fish waste.

Plant macro algae in your fish tank. The algae use the nitrates and phosphates in the water. The algae need sufficient light to thrive, which may mean increasing the lighting in the tank.

Add a refugium, a small tank which houses plants and other cleaner organisms which share the same water as the main tank. A refugium is usually one-tenth the size of the main tank and is brightly lit to enhance plant growth.

Tips & Warnings

  • Lighting the refugium on the opposite schedule as a planted aquarium reduces the pH fluctuation caused by plant release of CO2 at night.
  • The fresh salt water should be at least a day old to ensure that all the salt is completely dissolved and doesn't burn your fish.
  • Do not do a massive water change, greater than 30 percent. That changes the pH of the water suddenly, and your fish may go into shock. Either raise the tank pH or lower the pH of the aged salt water through chemical means prior to the change. You have to keep adjusting the pH until it is at its ideal level, usually between 7.6 and 8.2.

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