High nitrate nearly always comes from more food and waste going in than your filtration takes out. Cut back feeding, clean mechanical filters more often, and add an export such as a refugium if you need one. Bring it down over weeks, not days, and keep testing so you don’t overshoot to zero.
Where it comes from
Food goes in, fish turn it into waste, and bacteria turn that waste into ammonia, then nitrite, then nitrate. Anything that adds organic material adds nitrate. Bulk Reef Supply names overfeeding as the most common cause.
- Overfeeding, especially pellets and flake
- More fish than the filtration can keep up with
- Dirty filter socks, floss or sponges holding waste
- A skimmer that isn’t running well or needs cleaning
- Detritus in the sand or rockwork, or stirred up during maintenance
- Something that has died out of sight
- Tap water or poor RODI water used for top-up or water changes
Fixes, roughly in the order to try them
Start with the cheap, simple changes. Give each one a couple of weeks and watch the trend before adding the next.
- Feed less, or feed better. Feed only what’s eaten in a couple of minutes. Turn pumps off while feeding so less food ends up in the sump.
- Clean mechanical filtration often. Bulk Reef Supply suggests changing filter socks every 1–3 days. Waste trapped in a sock still breaks down into nitrate.
- Get the skimmer working well. Empty the cup and clean the neck regularly so it keeps pulling waste out.
- Check your source water. Test your RODI output with a TDS meter. See our guide to TDS and source water.
- Add an algae export. A refugium with macroalgae, a macroalgae reactor or an algae scrubber takes nitrate and phosphate up. You remove both when you harvest the algae.
- Consider carbon dosing. Dosing a carbon source feeds bacteria that use up nitrate and phosphate. It works, but it’s for experienced keepers, because it can strip nutrients too far.
Why water changes alone struggle
A water change removes the same percentage of nitrate as the percentage of water you change. A 10% change takes out about 10%. If nitrate is 40 mg/L, a 10% change takes it to about 36 mg/L, and the tank keeps making more.
Water changes help, especially a couple of larger ones to get started. But if the cause is still there, nitrate climbs back. Fix the input and the export first.
| After | Nitrate (no new nitrate made) |
|---|---|
| 1 change | 36 mg/L |
| 2 changes | 32.4 mg/L |
| 3 changes | 29.2 mg/L |
| 4 changes | 26.2 mg/L |
Real tanks keep producing nitrate between changes, so the true fall is slower than this.
Go slowly
A tank that has lived at high nitrate has corals adjusted to it. Red Sea warns that rapid nutrient changes can damage the algae inside corals. Aim to bring the level down over several weeks. Stop adding new fixes once you’re heading for your target.
Watch phosphate too. If you cut nitrate hard, phosphate can be left behind with nothing to balance it. Our phosphate guide covers that side.
Track it until it’s settled
Test nitrate and phosphate every 3–4 days while you’re fixing it, then weekly. Logging each result shows whether a change is working, and stops you adding a second fix before the first has had time to show.
Common questions
How fast can I lower nitrate?
Slowly. Several weeks is better than a few days. Sudden drops can stress corals that are used to the higher level.
Will a bigger skimmer fix high nitrate?
It can help, because a skimmer takes out waste before it becomes nitrate. It won’t fix overfeeding on its own.
Do nitrate-removing media work?
Some do, but they treat the symptom. Fix feeding and filtration first, so you aren’t relying on media forever.
Sources and limits
Last checked 12 October 2026. Ranges here come from the published sources below, which don’t all agree. They’re starting points to discuss with your own tank in mind, not guarantees for any coral.
- Bulk Reef Supply: 5 ways to lower nitrates
- Red Sea: Algae Management Program manual
- Randy Holmes-Farley, Reefkeeping: Reef Aquarium Water Parameters
Keep going
Track your nitrate as you fix it · Free Reef Companion · All calculators