If your fish tank shows an ammonia spike, confirm it with a second test, stop feeding, and do a 30% to 50% water change with dechlorinated water matched to the tank’s temperature. Repeat smaller changes daily until ammonia reads zero, then find what caused it. In an established tank the cause is almost always something you can track down: a dead fish, a filter cleaned too hard, a batch of new fish, or too much food.
How dangerous is my ammonia reading? #
It depends on pH and temperature as much as on the number from your test kit. Hobby kits measure total ammonia, which is a mix of two forms: ionized ammonium (NH4+) and un-ionized ammonia (NH3). The un-ionized form is the one that damages gills, and according to the University of Florida extension fact sheet Ammonia in Aquatic Systems, it’s roughly 100 times more toxic than ammonium. The share that’s un-ionized rises as pH and temperature rise.
UF/IFAS says tissue damage can occur any time un-ionized ammonia goes above 0.05 mg/L. Here’s what a total ammonia reading of 1.0 ppm works out to at different pH levels, using the fractions from the fact sheet’s Table 1:
| pH | Un-ionized fraction at 24°C (75°F) | Un-ionized fraction at 26°C (79°F) | 1.0 ppm total ammonia at 26°C |
|---|---|---|---|
| 7.0 | 0.0052 | 0.0060 | 0.006 mg/L NH3 |
| 8.0 | 0.0502 | 0.0574 | 0.057 mg/L NH3 |
| 8.2 | 0.0772 | 0.0880 | 0.088 mg/L NH3 |
The same kit reading is about ten times more dangerous at pH 8.0 than at pH 7.0. That’s why ammonia in a saltwater tank or an African cichlid tank, both usually above pH 8, is an emergency at levels a soft-water tetra tank might get through for a day or two. It isn’t safe in either case, though. Ammonia should read zero in any cycled tank.
To check your own tank, multiply your total ammonia reading by the fraction for your pH and temperature. If the answer is above 0.05, act now.
What should you do in the first hour? #
- Retest with a fresh sample. Rule out a dirty tube or a misread card. If you have a second kit, use it.
- Stop feeding. Food is the main source of ammonia in most tanks, and healthy fish can go several days without it.
- Look for a dead fish, snail or shrimp. Check behind rocks, inside decorations and around the filter intake. Remove rotting plant matter and uneaten food.
- Prepare replacement water. Dechlorinate it and match the temperature within a degree or two. For saltwater, match the salinity too.
- Change 30% to 50% of the water. That cuts the ammonia by the same percentage.
- Check the filter. If it stopped or was unplugged, restart it. If you just replaced all its media, put back any old media you still have.
- Increase surface agitation. More oxygen helps stressed fish and the bacteria that process ammonia.
- Retest in 12 to 24 hours, and repeat a smaller change if ammonia is still above zero.
Watch out for the pH jump after a water change #
If your tank has drifted to a low pH, say 6.4, and your tap water comes out at 7.8, a big water change raises the tank’s pH quickly. By the table above, that moves more of the existing ammonia into the toxic form. In that situation, do several smaller changes a few hours apart instead of one large one, and test pH along with ammonia. Low pH doesn’t make ammonia go away. It only keeps most of it in the less harmful form for the moment.
Should you use an ammonia-binding water conditioner? #
Several dechlorinators are sold as able to bind ammonia temporarily, which can buy time between water changes. They don’t remove ammonia, so the water change is still the fix. Some test kits keep reading bound ammonia as if it were free ammonia, so the number may not drop right after you dose one. Follow the product’s label, and don’t stack extra doses unless it says you can.
What causes an ammonia spike in an established tank? #
A cycled tank has enough bacteria to handle its normal waste. A spike means either the waste went up suddenly or the bacteria went down.
| Cause | Clues | Fix |
|---|---|---|
| Dead fish or invertebrate | Missing fish, cloudy water, smell | Remove it, water change |
| Overfeeding | Food on the substrate, cloudy water | Feed less, vacuum the substrate |
| Filter cleaned in tap water or all media replaced | Spike 2-5 days after filter maintenance | Rinse media only in tank water, keep some old media |
| Filter or power outage | Filter off for hours, dirty first flow on restart | Rinse the filter in tank water, extra water changes |
| Many new fish at once | Spike within a week of adding fish | Water changes, add fish in small groups |
| Medication that harms bacteria | Spike during or after treatment | Follow the medication label, water changes, daily tests |
| Chloramine in tap water | Ammonia shows up right after a water change | Use a conditioner that handles chloramine |
The last one catches a lot of people. Many water utilities disinfect with chloramine, which is chlorine bound to ammonia. A dechlorinator breaks that bond, and the ammonia left behind can show on your test until the filter bacteria process it. Test your tap water. If it reads ammonia on its own, your utility is probably using chloramine.
How long does an ammonia spike last? #
If the cause is gone and the bacteria are intact (you removed a dead fish, or it was one day of overfeeding), readings usually fall back to zero within a few days of water changes. If the bacteria took the hit (a filter cleaned in tap water, a medication, a long outage), the tank has to rebuild them. That can take weeks and looks like a small version of a new tank cycle. Keep ammonia down with water changes and watch for the nitrite rise that follows. Our fishless cycle readings guide shows what that recovery pattern looks like.
UF/IFAS also warns that fish can get sick weeks after an ammonia problem has been corrected, because the gill damage leaves them more vulnerable. Keep watching them for a while after the numbers look normal again.
How do you prevent the next ammonia spike? #
- Feed small amounts and remove anything not eaten within a few minutes.
- Count your fish every day.
- Rinse filter media in removed tank water, and never replace all of it at once.
- Add new fish a few at a time, with a week or two between groups.
- Test ammonia for a week after any big change: new fish, filter maintenance, medication.
- Keep regular water changes going. Our guide on how often to change aquarium water shows how to size them.
A record helps you connect a spike to its cause. Fish Tank Tracker logs ammonia, nitrite and nitrate along with pH and temperature, and each test has a notes field for entries like “cleaned filter” or “added 4 platies”. Its livestock journal records when each fish was added and marks losses, so when ammonia shows up on the chart you can scroll back and see what changed that week. It runs offline with no account, and the free version covers one tank.
For the full picture of what normal looks like, see our freshwater aquarium water parameters chart.
Frequently asked questions #
What ammonia level is dangerous for fish? #
Any reading above zero in a cycled tank is a warning. How dangerous it is depends on pH and temperature. At pH 8.0 and 79°F, 1 ppm of total ammonia is already above the 0.05 mg/L un-ionized level that UF/IFAS links to tissue damage, while at pH 7.0 the same reading is well under it. Act on any reading of 0.25 ppm or more.
Will an ammonia spike go away on its own? #
Sometimes, if the bacteria catch up, but the fish sit in damaging water while that happens. Water changes lower the level right away and don’t slow the bacteria, since the bacteria live on surfaces rather than in the water.
Do live plants help with ammonia? #
Yes. Fast-growing plants take up ammonia directly, and floating plants like frogbit or water lettuce are especially quick. They make spikes smaller but won’t clear a large one on their own.
Why does my tap water show ammonia? #
Most likely your water utility uses chloramine. When a conditioner breaks the chloramine apart, the ammonia part stays in the water until filter bacteria convert it. Use a conditioner rated for chloramine, and in a tank without an established filter, test after each water change.