What to do when ammonia spikes

What ammonia actually does to your fish

Ammonia (NH₃) is directly toxic to fish at the cellular level. It crosses gill membranes and interferes with enzyme function, oxygen transport, and the nervous system. Fish don't suffer quietly - you'll see them gasping at the surface, sitting motionless at the bottom, clamping their fins, or developing red-streaked skin and fins. At high concentrations it kills within hours. At low, chronic levels it causes lasting gill damage that makes fish permanently susceptible to disease even after the spike resolves.

Even a reading of 0.25 ppm is harmful. A lot of beginners see a "low" number and wait to see if it gets worse. Don't. Act at any reading above zero.

pH matters here too: ammonia exists in two forms in your tank water - ionised ammonium (NH₄⁺), which is relatively harmless, and un-ionised ammonia (NH₃), which is toxic. The higher your pH, the higher the proportion of the toxic form. A tank at pH 8.0 with 0.5 ppm total ammonia is significantly more dangerous than the same reading at pH 6.5. This is why you check pH alongside ammonia every time.

Step-by-step: what to do right now

1. Test properly - don't skip this

Grab a liquid test kit (not strips - they're too inaccurate for crisis diagnosis) and run ammonia, nitrite, and pH. The results tell you what stage of trouble you're in:

• Ammonia high, nitrite zero: the nitrogen cycle is absent or completely overwhelmed.
• Ammonia high, nitrite also high: the tank is mid-cycle, or the bacterial colony is under serious stress.
• Ammonia high, nitrite zero, nitrate present: your cycle exists but something is producing more ammonia than the bacteria can handle.

Log the exact reading with a timestamp. If you use App-aquatic, you can track each test result over time and see whether the spike is improving or worsening after each water change.

2. Do a large water change - now

A 30-50% water change is your immediate intervention. It doesn't fix the underlying problem, but it dilutes the ammonia fast enough to keep fish alive while you sort out the cause. A few things to get right:

• Match the temperature of the new water to within 1-2°C (2-4°F) of your tank. Cold shock on top of ammonia poisoning is brutal.
• Dechlorinate the new water before it goes in. Chlorine at the same time as an ammonia spike is adding injury to injury.
• If your reading is above 2 ppm, do a second 30% change a few hours later rather than one enormous change, which can destabilise water chemistry too quickly in some setups.

3. Stop feeding entirely

Every time you feed, uneaten food sinks and rots. Even food that fish eat produces ammonia as waste within hours. During an ammonia spike, the bacteria that would normally process that waste are already overwhelmed. Skipping feeding for 24-48 hours removes one ammonia source entirely. Healthy adult fish handle this without any problem - most species can fast for a week or more without harm. Fry and very small fish are the exception; they need feeding, but in tiny, controlled amounts with immediate removal of any uneaten food.

4. Don't touch the filter (seriously)

Your filter media - sponge, ceramic rings, bio-balls - is where the vast majority of your beneficial bacteria live. These bacteria are what convert ammonia to nitrite and then nitrite to nitrate. Cleaning media under tap water or replacing it during a spike doesn't help. It destroys your biological filtration and turns a manageable problem into a full tank crash.

If your filter is visibly blocked with detritus and flow is compromised, rinse the mechanical media (the coarse sponge or floss that catches particles) in a bucket of tank water you've siphoned out. Never use tap water, even briefly.

Why did the ammonia spike in the first place?

There's always a cause. Treating the spike without finding it means you'll be back in the same position in days or weeks. Here are the most common culprits:

Uncycled or partially cycled tank

This is the number one cause, especially in tanks under six weeks old. A brand-new tank has no established bacterial colony. Any fish waste immediately becomes toxic ammonia with nothing to process it. If your tank isn't fully cycled, you'll see ammonia spikes repeatedly until the cycle completes. Read the full guide on how to cycle a fish tank - cycling before adding fish is the single most important thing you can do.

Overfeeding

Feed only what fish consume in two minutes, once or twice a day. Anything that sinks and sits starts decomposing immediately. Most people significantly overestimate how much food their fish need - the fish will always act hungry, but that's not a reliable guide.

A dead fish or rotting plant matter

A single dead fish in a small tank can cause a measurable spike within 24 hours. Check every corner, behind equipment, and under decorations. Dying or heavily decaying plant leaves should be trimmed and removed. You'd be surprised how fast a large bunch of melting stem plants can push ammonia up.

Overstocking

Too many fish means more waste than your bacterial colony can process. If you're not sure whether your tank is overstocked, use the free stocking calculator to check your current setup against realistic bioload estimates. Keep in mind that fish grow - a tank stocked for juvenile fish will often be overstocked by the time those fish reach adult size.

Filter disruption

Cleaning too much of the filter at once, replacing all media simultaneously, running the filter dry, or a power cut that lasts several hours can all crash your bacterial colony. The bacteria that process ammonia need oxygen and a constant food source (ammonia). Disrupt either and the colony dies off.

Medications

Many antibacterial treatments kill beneficial bacteria as well as pathogens. If you've recently treated a disease, your cycle may be partially or fully disrupted. Always monitor ammonia closely after any medication course.

What most guides miss: the recovery phase

Getting ammonia back to zero is step one. The recovery phase - the days and weeks after - is where a lot of fishkeepers make new mistakes.

After a severe spike, your bacterial colony will be smaller than before. That means your tank's ability to process ammonia is reduced, even if readings look fine. During recovery: feed lightly, do more frequent water changes than usual (two or three times a week instead of one), and test every day. Don't add new fish for at least two to four weeks after ammonia returns to zero.

Watch fish closely for secondary infections. Ammonia damages gills and mucus membranes, opening the door to bacterial and fungal infections. Fish that survived a spike but look ragged, have red patches, or develop white fuzz in the days following may need treatment. The spike itself was the initial injury - secondary disease is often what finishes fish off.

Using ammonia detoxifiers

Products like Seachem Prime or API Ammo Lock temporarily detoxify ammonia by converting it to a less harmful form. They're genuinely useful as an emergency bridge - particularly overnight when you can't do a water change immediately, or when ammonia has spiked to dangerous levels in a short time. Use them as directed on the label.

What they don't do: they don't remove ammonia from the water, and they don't fix the underlying cause. Most standard test kits will still show ammonia present after detoxification (because the total ammonia level hasn't changed, only its form). Don't stop water changes because the detoxifier is in there. Keep changing water and testing.

Preventing future spikes

Once you've dealt with the immediate crisis, these habits will prevent a repeat:

• Test ammonia and nitrite weekly in any tank under three months old, and monthly in established tanks. Understand aquarium water parameters so you know what you're looking for.
• Feed conservatively. The "two-minute rule" is a practical guide - if there's food left after two minutes, you're feeding too much.
• Count your fish and know their adult sizes. What's appropriately stocked today may not be in six months.
• Never clean your entire filter at once. If you have multiple filter stages or media types, clean them in rotation over several weeks.
• When you treat for disease, monitor ammonia throughout and for two weeks after the treatment course ends.
• Understand the nitrogen cycle well enough to recognise what normal looks like for your tank, so abnormal readings stand out immediately.