Cycle and parameters
Nitrate is high no matter how often I change water
If nitrate is back above 40 ppm within days of a change, dilution is not your lever — the input is. Test your tap water first, because a supply that already carries nitrate puts a floor under everything you do. After that it is feeding, stocking, and how much waste is sitting in the substrate.
Act this week
Test these first
Before anything goes into the tank and before anything comes out of it, get these numbers. They cost fifteen minutes and they decide everything that follows.
- Nitrate — the nitrate ion — note that drinking water standards are written as nitrate-nitrogen, which is a different number
- Ammonia — total ammonia — the ammonium ion plus free ammonia, added together
What is usually behind it
| Cause | How to confirm | Time to resolve |
|---|---|---|
| Nitrate already present in the tap water | Fill a test tube from the tap, treat nothing, and run the nitrate test on it. Shake bottle 2 hard for the full time first. Your utility's annual water quality report will also list it. | Immediate as a diagnosis; the tank baseline shifts over four to six weeks |
| Waste accumulated in the substrate | Push a gravel vacuum into an undisturbed corner and watch what comes up. If the water in the tube goes brown and stays brown, there is a reservoir down there. | Three weeks to work through the bed, with readings improving each week |
| Feeding more than the tank consumes | Watch a feeding to the end. If anything is still on the substrate after two minutes, the portion is too large. Flake that lands and stays is nitrate on a delay. | Two to four weeks for the baseline to fall |
| More fish than the volume carries | Nitrate returns to its previous level within three or four days of a 50% change, feeding is already modest, and the substrate is clean. At that point the arithmetic is the answer. | Ongoing — this is a standing condition rather than an event |
| Chemical media assumed to be handling it | There is carbon or a resin in the filter and it has been there for months. Neither removes nitrate in any quantity, and both stop doing their actual job once exhausted. | No direct effect on nitrate; corrects the expectation immediately |
Nitrate already present in the tap water
How to confirmFill a test tube from the tap, treat nothing, and run the nitrate test on it. Shake bottle 2 hard for the full time first. Your utility's annual water quality report will also list it.
What to doRecalculate what a water change can achieve. If the tap reads 20 ppm, a 50% change on a 60 ppm tank lands at 40 ppm, not 30. Plants and reduced input become the main tools; larger changes stop being one.
Time to resolve: Immediate as a diagnosis; the tank baseline shifts over four to six weeks
Waste accumulated in the substrate
How to confirmPush a gravel vacuum into an undisturbed corner and watch what comes up. If the water in the tube goes brown and stays brown, there is a reservoir down there.
What to doVacuum a third of the substrate at each water change, rotating sections, until the whole bed has been through. Doing it all in one session releases too much at once and clouds the tank badly.
Time to resolve: Three weeks to work through the bed, with readings improving each week
Feeding more than the tank consumes
How to confirmWatch a feeding to the end. If anything is still on the substrate after two minutes, the portion is too large. Flake that lands and stays is nitrate on a delay.
What to doFeed once a day, only what is gone in about a minute, and skip one day a week. This is the single most effective change available to most tanks and it costs nothing.
Time to resolve: Two to four weeks for the baseline to fall
More fish than the volume carries
How to confirmNitrate returns to its previous level within three or four days of a 50% change, feeding is already modest, and the substrate is clean. At that point the arithmetic is the answer.
What to doEither raise the water change frequency permanently, add substantial fast-growing plant mass, or reduce the stocking. A tank can be run at any of the three; it cannot be run at none of them.
Time to resolve: Ongoing — this is a standing condition rather than an event
Chemical media assumed to be handling it
How to confirmThere is carbon or a resin in the filter and it has been there for months. Neither removes nitrate in any quantity, and both stop doing their actual job once exhausted.
What to doReplace chemical media on the manufacturer's interval and stop counting it as nitrate control. Water changes, plants and smaller inputs are what move this number.
Time to resolve: No direct effect on nitrate; corrects the expectation immediately
What not to do
These are the moves that make this particular problem worse, and they are all common. Most of them come from wanting to do something substantial when the right answer is something small.
- Do not do one enormous water change to reset the number. Dropping a long-standing high nitrate tank abruptly changes far more than nitrate, and stepped changes over several days are kinder.
- Do not rely on activated carbon to lower nitrate. It does not.
- Do not stop testing because the number is depressing. The trend across weeks is the useful part.
- Do not add more filtration and expect the number to fall. Filtration produces nitrate; it does not remove it.
Keeping it from coming back
High nitrate is a slow accumulation, so prevention is a set of small standing habits rather than an intervention. Feed once a day and skip a day each week. Vacuum a section of substrate at every water change so no part of the bed goes more than a month without attention. Keep a few fast-growing plants even in a tank you do not think of as planted. And test the tap once a year, because the number that arrives at your house is part of the arithmetic and it does change.
Reference for this page
Equipment involved
- Test Kitsknowing the number instead of guessing it
- Filter Media & Cartridgeswhat actually does the filtering, and when it gets replaced
- Filtersturning water over and housing the bacteria
Related problems
Act this week
Watch it
Watch it
Published sources
- US EPA — National Primary Drinking Water Regulations
- US EPA — Nutrient Pollution
- USGS Water Science School — Nitrogen and Water
Published figures on this page come from the documents above. Where a value depends on species, pH or temperature, that is stated rather than averaged into a single number.