Nitrite vs Nitrate: The Aquarium Chemistry You Can’t Ignore
Nitrite and nitrate sound similar, but they affect your fish very differently. This guide explains what each one does, which is dangerous, safe levels to aim for, and how they fit into the nitrogen cycle.
Quick Summary
Nitrite and nitrate sound similar, but they affect your fish very differently. This guide explains what each one does, which is dangerous, safe levels to aim for, and how they fit into the nitrogen cycle.
Key takeaways
- Nitrite is the urgent danger: it can poison fish even at low levels, so aim for 0 ppm at all times.
- Nitrate is much less toxic, but it builds up over time; keep it low with regular water changes and avoid chronic high readings.
- In a healthy tank, ammonia turns into nitrite, then nitrate-seeing nitrite usually means the tank isn't fully cycled or the filter bacteria were disturbed.
- Test for nitrite and nitrate separately with a liquid test kit; if nitrite shows up, stop feeding for a day or two and do partial water changes until it reads zero.
- Prevent spikes by not overstocking, not overfeeding, and keeping the filter running with media rinsed only in old tank water (not tap water).
Nitrite vs Nitrate (and why your test kit has both)
Nitrite and nitrate look like cousins on a test chart, but they behave totally differently in a tank. One can knock fish over fast, the other is more like "slow stress" that builds up if you ignore it.
If you've ever cycled a tank, dealt with mystery fish deaths, or wondered why your water looks fine but fish act weird, this is the chemistry piece you can't skip.
Quick translation
Nitrite (NO2-) = immediate problem during cycling or after a filter crash. Nitrate (NO3-) = end product that creeps up over time and tells you how "used" your water is.
The nitrogen cycle in plain hobbyist language
Fish poop, leftover food, dead plant bits-anything rotting turns into ammonia. Your filter (and every surface in the tank) grows bacteria that eat that ammonia and turn it into nitrite. Then another group of bacteria eats nitrite and turns it into nitrate.
So the usual path looks like this: ammonia → nitrite → nitrate. Most "new tank problems" are just one of those steps not keeping up yet.
- Ammonia (NH3/NH4+) comes from waste and decay
- Nitrite (NO2-) shows up when ammonia-eaters are working but nitrite-eaters aren't caught up yet
- Nitrate (NO3-) builds once the cycle is established-and keeps building until you remove it
Where the bacteria actually live
Mostly in your filter media, not in the water. That's why swapping filters, over-cleaning media, or losing power to the filter can cause a nitrite spike even in an "old" tank.
Nitrite: the one that can hurt fish fast
Nitrite is dangerous because it messes with oxygen transport in the blood (you'll hear it called "brown blood disease"). The tank can have plenty of oxygen and fish still act like they can't breathe.
If you see nitrite on a test after a tank has been running a while, treat it like an alarm. Something disrupted your biofilter or suddenly increased the bioload.
- Common nitrite spike triggers: new tank cycling, overfeeding, adding too many fish at once, filter media rinsed in tap water, filter off for hours, medication that hits bacteria, big substrate clean that stirs up gunk
- Fish often show it as: rapid breathing, hanging near the surface or filter outflow, lethargy, clamped fins, acting "spooked," sometimes pale/brownish gills
Target level for nitrite
Aim for 0 ppm nitrite. If your kit shows anything above 0, assume fish are being stressed and start fixing it the same day.
What to do if you detect nitrite
This is one of those moments where you don't overthink it. Your job is to dilute the nitrite, support the biofilter, and stop adding more waste than the tank can process.
- Do a big water change (30-70% depending on how high the reading is). Match temp roughly and dechlorinate.
- Feed lightly (or skip a day). Less food = less ammonia = less nitrite downstream.
- Check the filter: is it running full flow? Any media replaced recently? Any cartridges thrown out?
- Add extra aeration. Nitrite issues often show up as breathing trouble, so an air stone helps fish cope.
- Consider adding established media (a sponge, ceramic rings) from a healthy tank if you have one.
- Test daily until nitrite is back to 0 and stays there.
Salt trick (works, but don't wing it)
Chloride competes with nitrite uptake at the gills. In a pinch, adding a small amount of plain aquarium salt can reduce nitrite toxicity for many freshwater fish. That said, some fish/plants hate salt (loaches, some catfish, many plants), so research your stock before you reach for it.
Nitrate: the slow-burn indicator of maintenance
Nitrate is what you get after your biofilter finishes the job. It's much less toxic than nitrite, but it's not "free." If it climbs and stays high, fish can get stressed, colors can dull, growth slows, and algae often gets a vote.
Nitrate is also a really handy reality check. If your nitrate is rising week after week, the tank is producing more waste than your routine is exporting.
Why nitrate keeps going up
Your filter doesn't remove nitrate-it creates it. The main ways nitrate leaves most aquariums are water changes, plant uptake, and (in specific setups) denitrification.
What nitrate level should you aim for?
You'll hear a bunch of numbers online because different fish, tap waters, and tank styles vary. Here's a practical way to think about it: keep nitrate low enough that your fish look and act normal, and stable enough that you're not swinging parameters wildly.
- Community freshwater tanks: many people aim for under ~20-40 ppm
- Sensitive fish (some shrimp, wild-caught species, fry): often happier lower
- If your tap water already has nitrate: your "best possible" number may be higher than you want-focus on consistency and dilution
Don't chase a perfect zero nitrate
A planted tank can run very low nitrate, but in many tanks "0 nitrate" on a hobby kit can also mean the cycle isn't established, the test was done wrong, or the tank is starving plants and then algae takes over anyway. Use the number as a trend, not a trophy.
How to lower nitrate (what actually works)
Water changes are the blunt instrument, and they're reliable. Plants are the quiet long-term helper. Everything else is situational.
- Bigger or more frequent water changes (the fastest fix)
- Feed less / reduce waste (stop nitrate at the source)
- Add fast-growing plants (floating plants, stem plants) or a pothos cutting with roots in the water
- Rinse mechanical gunk out of filter sponges in old tank water (less decaying sludge = less nitrate production over time)
- Reduce stocking if the tank is simply overloaded
- Use nitrate-removing media or resins if you need a temporary assist (handy, but don't let it replace a routine)
My favorite "lazy" nitrate reducer
Floating plants. They suck up nitrate, shade the tank a bit, and they're a nice safety net if you miss a water change. Just keep them from blocking all surface movement.
Reading your test results like a hobbyist (not a chemist)
Numbers matter, but patterns matter more. A single test can be thrown off by old reagents, bad shaking, lighting, or just misreading the color. If something looks off, retest and look for a trend over a few days.
Common test kit mistakes (I've done most of these)
- Not shaking nitrate bottle #2 long enough (liquid kits often need aggressive shaking)
- Reading the color under warm/yellow room light instead of daylight
- Letting the test sit way longer than the instructions before reading
- Cross-contaminating caps/tubes between tests
- Using expired reagents (especially nitrate reagents that cake up)
Use nitrate as your maintenance gauge
If nitrate climbs from 20 → 40 ppm between water changes, you can predict your routine isn't keeping up. Adjust the schedule and watch the next couple weeks. That's way more useful than obsessing over a single reading.
How nitrite and nitrate show up during cycling
During a typical cycle, you'll first see ammonia rise. Then nitrite appears (sometimes it shoots up and sits there for what feels like forever). Once the second bacteria group catches up, nitrite drops to zero and nitrate starts climbing.
- Early cycle: ammonia up, nitrite 0, nitrate 0-low
- Mid cycle: ammonia starts dropping, nitrite rises, nitrate starts showing
- Late cycle: ammonia 0, nitrite 0, nitrate rising
The "looks cycled" trap
Some people test only ammonia and see 0, then add fish. But if nitrite isn't at 0 yet, the tank isn't ready. Always test ammonia + nitrite before calling it.
Why you can get nitrite in an established tank
A mature tank doesn't magically prevent nitrite forever. It just has enough bacteria to handle the usual load. If you suddenly change the load or damage the colony, nitrite can pop back up.
- Big filter change (new cartridge, tossing bio media)
- Deep-cleaning everything at once (filter + substrate + decorations)
- Long power outage or a filter left off overnight
- Heavily medicating in the display tank
- Adding a bunch of new fish at once
- A dead fish/snail stuck in decor or filter intake
Avoid the "clean reset" mistake
If you want a clean-looking tank, do it in stages. Vacuum one section this week, rinse one sponge next week. The goal is to remove gunk without wiping out your biofilter's capacity.
Fast reference: what to do at each reading
If nitrite is...
- 0 ppm: great-keep your routine steady
- 0.25-0.5 ppm: water change, feed light, retest tomorrow
- 1+ ppm: big water change now, add aeration, investigate filter/bioload immediately, test daily
If nitrate is...
- Low and stable: your routine matches your stocking
- Rising steadily: increase water change volume/frequency or add plants/reduce feeding
- Very high: do larger changes over a few days rather than one massive swing, especially with sensitive fish
Watch for big parameter swings
Huge water changes can be totally fine, but if your tap water is very different (temperature, hardness, pH), doing it in chunks can be gentler than one giant reset.
FAQ
Can plants "fix" nitrite?
Plants can use nitrogen, but they're not your emergency nitrite solution. If nitrite shows up, you still handle it with water changes and fixing the biofilter issue. Plants help more with nitrate in the long run.
Why is nitrate high but nitrite is zero?
That's the normal "cycled tank" pattern. The biofilter is processing waste correctly, and nitrate is simply accumulating because it's not being exported fast enough.
My nitrate is always 0-should I worry?
Maybe, maybe not. In a heavily planted tank with light stocking, 0 can be real. But if it's a non-planted tank and you also get weird algae or fish issues, double-check your nitrate test method and verify the cycle with an ammonia + nitrite test.
Do water conditioners remove nitrite or nitrate?
Most conditioners mainly handle chlorine/chloramine. Some products claim to "detox" ammonia/nitrite temporarily, which can help in emergencies, but they don't replace water changes or an established biofilter. For nitrate, conditioners usually don't remove it-you export nitrate by removing water or by plant uptake/other specialized methods.
A simple weekly routine that keeps both in check
If you want the low-drama version of fishkeeping, build a routine that prevents nitrite spikes and keeps nitrate from creeping up.
- Test nitrate weekly at first (and nitrite anytime fish act off).
- Do a consistent water change schedule you can actually stick to.
- Feed what gets eaten quickly-adjust based on leftovers, not vibes.
- Rinse filter sponges/media in old tank water when flow drops (not on a fixed calendar).
- Avoid replacing all media at once; keep some old media running if you must swap things.
If you only remember one thing
Nitrite = emergency, treat immediately. Nitrate = maintenance signal, adjust your routine.
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