Testing
My test strips and my liquid kit give different answers
Disagreement between two kits is a procedure question far more often than a product question. Check expiry dates, check that reagents were shaken properly, and run both tests on the same sample at the same time. Until you know which is right, act on the more alarming reading — a water change is the safe response to either answer.
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.
- Ammonia — total ammonia — the ammonium ion plus free ammonia, added together
- Nitrite — nitrite ion, the intermediate product between ammonia and nitrate
- Nitrate — the nitrate ion — note that drinking water standards are written as nitrate-nitrogen, which is a different number
- pH and KH — acidity of the water, plus the carbonate buffer that resists changing it
What is usually behind it
| Cause | How to confirm | Time to resolve |
|---|---|---|
| An expired or humidity-damaged kit | Check the date on the strip tube and on each reagent bottle. Strips are especially vulnerable — a tube left open, or stored in a bathroom or a cabinet under the tank, degrades in weeks rather than months. | Same day once a current kit is in hand |
| Reagent not shaken enough | This is the classic nitrate failure. Bottle 2 in a liquid nitrate kit contains a suspension that settles hard, and under-shaking produces a reading far below the truth. | Ten minutes |
| Reading the color under the wrong light | The two kits agree in daylight at a window and disagree under a lamp in the evening. Warm indoor light shifts yellow-to-orange scales badly, which is most of the nitrate card. | Immediate, once you fix where you read the tube |
| The two kits are not measuring the same thing | Read both labels. Some strips report alkalinity where you expected carbonate hardness, or nitrate-nitrogen where the liquid kit reports the whole nitrate ion — a difference of roughly a factor of four and a half. | Immediate, once the units are understood |
| Sample handling differences | One sample came from the surface near the outflow and the other from mid-water, or one tube sat for an hour before it was read. | Immediate, from the next test onward |
An expired or humidity-damaged kit
How to confirmCheck the date on the strip tube and on each reagent bottle. Strips are especially vulnerable — a tube left open, or stored in a bathroom or a cabinet under the tank, degrades in weeks rather than months.
What to doReplace whichever is out of date and re-run both on the same sample. Keep strips capped tightly, dry, and away from the tank; take one out and close the tube immediately.
Time to resolve: Same day once a current kit is in hand
Reagent not shaken enough
How to confirmThis is the classic nitrate failure. Bottle 2 in a liquid nitrate kit contains a suspension that settles hard, and under-shaking produces a reading far below the truth.
What to doShake bottle 2 vigorously for the full time the instructions specify, then shake the filled tube for its full time as well. Re-run the test — a big jump between the two attempts identifies the problem conclusively.
Time to resolve: Ten minutes
Reading the color under the wrong light
How to confirmThe two kits agree in daylight at a window and disagree under a lamp in the evening. Warm indoor light shifts yellow-to-orange scales badly, which is most of the nitrate card.
What to doRead every test against the white part of the card, in daylight, away from the tank. Standardize where you do it and the disagreements often vanish.
Time to resolve: Immediate, once you fix where you read the tube
The two kits are not measuring the same thing
How to confirmRead both labels. Some strips report alkalinity where you expected carbonate hardness, or nitrate-nitrogen where the liquid kit reports the whole nitrate ion — a difference of roughly a factor of four and a half.
What to doCompare like with like, converting where necessary. This one is not a fault in either kit and no amount of retesting will make the numbers match.
Time to resolve: Immediate, once the units are understood
Sample handling differences
How to confirmOne sample came from the surface near the outflow and the other from mid-water, or one tube sat for an hour before it was read.
What to doDraw one sample into a clean container and fill both tests from it, at the same moment, and read both at the times their instructions specify.
Time to resolve: Immediate, from the next test onward
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 average the two results. One of them is wrong and the average is wrong too.
- Do not assume the liquid kit is right because it costs more. A liquid kit used carelessly is less accurate than a fresh strip used properly.
- Do not dismiss an alarming reading as a bad test without re-running it. Change water while you find out.
- Do not store test kits on top of the tank hood. Heat and humidity are what kill them.
Keeping it from coming back
Testing is a procedure, and procedures benefit from being boring. Write the open date on every strip tube and reagent bottle the day you break the seal. Keep them in a cool dry cupboard away from the tank. Test at the same time of day, from the same spot in the tank, against white paper in daylight, and write the numbers in the same notebook every time. Almost every disagreement this page describes disappears under that routine.
Reference for this page
Equipment involved
- Test Kitsknowing the number instead of guessing it
- Water Conditioners & Additivesmaking tap water safe to put in a tank
- Filter Media & Cartridgeswhat actually does the filtering, and when it gets replaced
Related problems
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Published sources
- US EPA — National Primary Drinking Water Regulations
- USGS Water Science School
- 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.