Why Nitrate Keeps Climbing

Written 2026-10-17

Ammonia and nitrite get handed off to the next step by filter bacteria and end up near zero, but nitrate is the last link in that chain — there’s nothing to hand it off to. Running a 60cm standard tank with 10 neon tetras and no water changes in Water Kiwoogi, cycling itself finishes by day 24 (ammonia and nitrite drop to near-zero test-kit readings), yet nitrate keeps rising — 17.5ppm by day 30, 41.5ppm by day 60, 57.1ppm by day 90, past the 49ppm safety line.

Why nitrate doesn’t drop — the last link in the chain

Two bacteria populations live in the filter: Nitrosomonas turns ammonia into nitrite, and Nitrobacter turns nitrite into nitrate. Both ammonia and nitrite shrink because something hands them off to the next step, so once cycling finishes they stay near zero. But nothing in a normal, oxygen-rich filter breaks nitrate down further. The only ways to bring nitrate down are water changes (swapping the water out), plant uptake, and the slow anaerobic denitrification that happens deep in thick substrate. Water change timing is covered in "How often should you change the water," and using denitrification to go without water changes is in "Can you run a tank with no water changes."

In numbers — ammonia and nitrite stop, nitrate doesn’t

The same tank (60cm standard, mid-size canister filter, 10 neon tetras, no plants, no water changes) was run for different lengths of time. Cycling finishes by day 24, and ammonia and nitrite stay near zero after that — but nitrate keeps climbing the whole time.

How plants and water changes change the rate

Adding plants or water changes to the same setup and comparing nitrate at day 90: with neither, it reached 57.1ppm. Adding a weekly 25% water change brought it down to 26ppm. Skipping water changes but adding a heavy planting (10 each of floating plants, Vallisneria, and Hygrophila) brought it down to just 4.4ppm by day 90 — close to tap water’s own nitrate level (about 4.4ppm on a test kit).

In this tank, heavy planting held nitrate lower than the weekly water change did. Plants pull nitrate straight out of the water, while a water change only dilutes it — the time between changes still lets nitrate build back up. That said, plants take more work (light, CO₂, trimming), and a water change also replenishes KH (buffering capacity) at the same time. It’s not really either-or — running both together is the safer bet.

What else changes how fast nitrate climbs

Feeding more means fish put out more nitrogen, which speeds up nitrate buildup too — see "What happens when you overfeed" for the numbers. A thick substrate can let some nitrate leak away through denitrification, but without water changes that setup still runs into a different problem: KH keeps dropping until pH collapses — see "Can you run a tank with no water changes" and "Why pH crashes when KH runs out" together for that.

So a climbing nitrate reading isn’t a sign something’s wrong

Nitrate creeping up day by day actually means the filter is doing its job — steadily oxidizing ammonia and nitrite all the way through. The catch is that someone has to clean up that end product. Keep up a weekly water change, add plants, and skip the extra pinch of food, and nitrate stays bounded under the safety line. It’s worth plugging in your own tank’s size, filter, stocking, and water-change schedule and running it yourself to see how much nitrate builds up.

The button below runs the 60cm standard tank, mid-size canister filter, 10 neon tetras, no water changes (90 days) tank from this guide.

The numbers in this guide come from the Water Kiwoogi simulator model; real tanks can differ.