Nitrate

What it is

Nitrate is the last stop of the nitrogen cycle in an ordinary freshwater tank. Ammonia becomes nitrite, nitrite becomes nitrate, and there the chain effectively ends, because the bacteria that would break nitrate down further need low-oxygen conditions that a well-run aquarium deliberately does not have. So nitrate accumulates. It goes up between water changes and down when you change water, and the shape of that sawtooth is a fairly honest record of how you have been maintaining the tank.

Why it matters

Two things make nitrate worth watching. It is far less acutely toxic than ammonia or nitrite, so a high reading is a chronic problem rather than an emergency — but it is fuel, and a tank sitting at a high nitrate number is a tank that will grow algae given any light at all. There is also a units trap here that catches almost everyone: EPA sets the drinking water limit at 10 mg/L measured as nitrate-nitrogen, while the aquarium kit on your shelf reports the whole nitrate ion. The two scales differ by a factor of about 4.4, so a lab result and a hobby kit can describe the same water with very different numbers. Check which one your kit prints before you compare anything to a published figure.

What each band means

How to test it

1. Shake bottle 2 hard for the full time the instructions give — thirty seconds is not a suggestion. The reagent contains a suspension that settles into a cake at the bottom of the bottle.
2. Then shake the filled tube for the full time as well. Under-shaking at either step is why nitrate reads low far more often than it reads high.
3. Test right before a water change. That is the peak of the sawtooth and the number that tells you whether your interval is right.
4. Also test your tap water once. Some municipal and well supplies arrive with meaningful nitrate already in them, which changes what a water change can achieve.
5. Read against white in daylight. The nitrate card runs yellow through orange to red and is the hardest of the four to judge under warm indoor light.

How to move it

1. Change water on a schedule you actually keep. A steady 25% weekly beats an occasional 50%, because the tank never gets far from its own baseline.
2. Vacuum the substrate while you change water rather than just siphoning open water. Most of the stored waste is in the top layer of gravel or sand.
3. Feed less and feed less often. Uneaten food is nitrate that has not happened yet.
4. Grow plants, especially fast stem plants and floaters. They take up nitrogen directly and are the only part of an ordinary tank that exports it without a bucket.
5. Reconsider stocking if the number climbs back within days of every change. At some point the tank is telling you it holds more fish than its water volume can carry.

Where the reading goes wrong

• Not shaking bottle 2 enough. This one error accounts for an enormous share of reassuringly low nitrate readings.
• Comparing a hobby nitrate-ion result to a lab or municipal nitrate-nitrogen figure without converting, and concluding the tank is fine when it is not, or vice versa.
• Testing after a water change and using that as the weekly record. Record the pre-change number; it is the one that varies with your habits.
• Assuming a nitrate reading of zero in a stocked tank is good news. In a tank with fish and no plants it usually means the test was under-shaken or the kit has expired.

Problems that start here

• Act this week

  Nitrate is high no matter how often I change water

• Act this week

  The water has turned green and I cannot see through it

• Watch it

  Hard green dots are growing on the glass

• Act this week

  Black or dark red tufts are growing on my plants and decor