Ammonia in Fish Tank Water: Why pH Decides How Toxic It Is

Ammonia in fish tank water is one of the most common ways fish get poisoned in captivity, and one of the easiest to miss. It doesn't cloud the water or smell at the levels that hurt fish, and the number on your test kit doesn't tell the whole story. Two tanks can both read 1 ppm. In one the fish are fine, and in the other they're suffering gill damage. The difference is pH and temperature.

Here's where ammonia comes from, what your test kit is actually measuring, how to tell when a reading is dangerous, and what to do when it is.

Where ammonia in a fish tank comes from

Ammonia is the main waste product of fish metabolism. Most of it leaves the fish by diffusing across the gills into the water, with smaller amounts in urine (UF/IFAS FA-16). Even a fish that isn't eating produces some. On top of that, uneaten food, dead plant matter and a dead fish hidden behind the rockwork all break down into ammonia.

In a cycled tank, a biofilm of nitrifying microbes on the filter media and other surfaces converts that ammonia to nitrite and then nitrate about as fast as it's produced. Ammonia shows up on a test when something upsets that balance:

• The tank isn't cycled yet. New tanks and fresh quarantine tanks have no biofilter.
• The load jumped. A big batch of new fish, heavy feeding or a death.
• The biofilter took a hit. A power outage, a filter cleaned under tap water, all the media replaced at once, or certain medications.
• The water change added it. Treating chloraminated tap water releases ammonia. See the chloramine guide for the math.

For how the biofilter works and how to build one, see The Aquarium Nitrogen Cycle.

Two forms of ammonia, and why pH matters

In water, ammonia exists in two forms that constantly convert back and forth. Un-ionized ammonia (NH₃) is the toxic one. Ionized ammonium (NH₄⁺) is far less toxic. UF/IFAS puts NH₃ at roughly 100 times more toxic (FA-16). Together they're called total ammonia, or total ammonia nitrogen (TAN) when the result is expressed as nitrogen.

The split between the two depends mainly on pH and temperature. The higher either one goes, the more of the total is in the toxic NH₃ form. pH has by far the bigger effect.

Percent of total ammonia in the toxic form

Calculated with the Emerson et al. (1975) equation used in UF/IFAS FA-16. The pH 7.0 and 8.0 rows match FA-16's published table. FA-16 lists pH in 0.2-unit steps from 7.0, so the 6.5, 7.5 and 8.5 rows are calculated with the same equation.

Each step up in pH multiplies the toxic share about threefold. A full unit multiplies it about tenfold.

When is ammonia in fish tank water dangerous?

UF/IFAS states that tissue damage can occur any time un-ionized ammonia is above 0.05 mg/L, that the damage grows as the level rises, and that sensitive fish typically die around 2.0 mg/L (FA-16). Treat these as rough guides. UF/IFAS multiplies the total ammonia nitrogen reading by the table's fraction, so whether your kit reports as nitrogen (NH₃-N) or as ammonia shifts the answer by about 20%. That's small next to the tenfold jump from a single pH unit. Long before it kills, low-level ammonia stresses fish, damages gills, slows growth and makes them more prone to bacterial infections.

To turn a test-kit reading into a risk, multiply the total ammonia reading by the percentage in the table. Here's an example at 79°F:

• 1.0 ppm total ammonia at pH 7.0: 1.0 × 0.60% = 0.006 mg/L NH₃. That's well under 0.05 and not an emergency yet, though it is a warning.
• 1.0 ppm total ammonia at pH 8.0: 1.0 × 5.74% = 0.057 mg/L NH₃. That's already past the damage threshold.

Put another way, at 79°F the total ammonia it takes to reach 0.05 mg/L of NH₃ is about 8.3 ppm at pH 7.0, 2.7 ppm at pH 7.5, 0.87 ppm at pH 8.0 and only 0.31 ppm at pH 8.5. Hard, alkaline tanks have very little room for error. Soft, acidic tanks have more, but they also tend to have weaker biofilters, since nitrification slows below pH 7. The GH vs KH guide explains why.

Don't read this as permission to ignore ammonia in a low-pH tank. Any steady ammonia reading in an established tank means something is wrong with the filter, the feeding or the stocking.

What your ammonia test kit actually measures

Most hobby ammonia kits measure total ammonia, not the toxic fraction (FA-16). Some kits report the result as ammonia-nitrogen (NH₃-N) and others as total ammonia (NH₃/NH₄⁺). The numbers differ by a factor of about 1.2 to 1.3, so check your kit's instructions. To use the table above, you also need a pH reading and a thermometer.

A few things throw off the reading:

• Ammonia binders and water conditioners. Nessler-type kits read falsely high when a binder or many conditioners are in the water, and after formalin treatments (FA-16). Salicylate kits are fine if you read them right away, at the time the instructions give and not later. Left sitting, the reagent slowly breaks the bound ammonia loose and the color creeps up (Seachem).
• Bound ammonia still counts as total ammonia. A binder holds ammonia in a less toxic form for about a day or two, by one manufacturer's account (Seachem). Your biofilter still has to process it.
• Timing. Test before a water change, then again the next day, rather than minutes after dosing conditioner.

Signs of ammonia stress

• Fish gasping at the surface or hanging near the filter outflow
• Fast gill movement, or gills that look red or inflamed
• Lethargy, clamped fins, loss of appetite
• Fish lying on the bottom, or darting erratically
• In chronic cases, cloudy eyes, frayed fins and recurring infections

None of these is unique to ammonia. Low oxygen and nitrite poisoning look much the same. That's why you test before you treat.

Ammonia emergency checklist

If ammonia reads high and the fish are showing stress:

1. Test pH and temperature along with ammonia, and work out the toxic fraction from the table above.
2. Change water now, but check the pH gap first. UF/IFAS suggests a 25% to 50% change (FA-16). Dose conditioner for the full tank volume, and match the new water's temperature. Before you start, compare your tap's pH with the tank's. If the tank has drifted well below the tap (common in an old, soft-water tank whose KH has run out), a big change can raise the pH faster than it dilutes the ammonia. For example, at 79°F, 4 ppm of total ammonia at pH 6.4 is about 0.006 mg/L NH₃. Halve it with a 50% change, let the pH settle at 7.6, and the remaining 2 ppm is about 0.047 mg/L, roughly eight times more toxic. In that situation, dose the ammonia binder first, then do smaller changes (around 25%) a few hours apart, dosing binder each time. For very high readings in any tank, two changes a few hours apart are safer than one massive one.
3. Add an ammonia-binding conditioner to buy time while the biofilter catches up.
4. Stop or cut feeding for 1 to 3 days (FA-16). Fish handle a short fast far better than ammonia.
5. Find the source. Look for a dead fish, rotting food or a stopped filter. Ask whether the filter media was recently replaced or rinsed in tap water.
6. Don't raise the pH. Adding buffer to a tank with ammonia makes more of it toxic. If pH needs correcting, do it after the ammonia is gone.
7. Increase aeration. Ammonia-stressed gills take in oxygen poorly, and UF/IFAS advises acting if dissolved oxygen falls below 5 mg/L (FA-16). Heavy aeration also drives off CO₂, which can nudge pH up, so recheck pH an hour or so later.
8. Test daily until ammonia and nitrite both read zero for several days. Nitrite often spikes next as the biofilter recovers. See nitrite in freshwater aquariums.

Ammonia when new fish arrive

Shipped fish ride overnight in a sealed bag with their own ammonia building up. The bag is protected by its chemistry: CO₂ from the fish lowers the pH, which keeps most of that ammonia in the less toxic form. When the bag is opened, CO₂ escapes, pH rises and the toxic fraction climbs. Mixing in higher-pH tank water does the same thing. That's why the binder goes in the moment the bag is opened, after a 15 to 30 minute float to match temperatures and before the 2 to 3 hours of measured flow that bring the fish over to your water. How Long Can Fish Survive in a Bag? has the full steps and works through the numbers, including why a shipment delayed well past overnight is riskier to acclimate. A new batch of fish also raises the ammonia load in your tank overnight. Wait a couple of hours or more after acclimating before offering food (a new fish often won't eat until the next day), then feed lightly for the first several days and test daily. If you're adding large fish to a tank that has only held small ones, the biofilter needs time to catch up.

FAQ

What level of ammonia is safe in a fish tank?

In an established tank, total ammonia should read zero. The toxic fraction starts causing tissue damage above about 0.05 mg/L of un-ionized ammonia. How much total ammonia that takes depends on pH and temperature: under 1 ppm at pH 8.0, and several ppm at pH 7.0.

Does a water conditioner remove ammonia?

No. Some conditioners bind ammonia temporarily, which lowers its toxicity for a day or so. The biofilter or a water change removes it.

Why does my tap water test positive for ammonia?

If your utility uses chloramine, neutralizing it releases ammonia. Some source waters also contain a little ammonia. A cycled tank handles it.

How long does an ammonia spike last?

In a tank with a damaged biofilter, it can last days to a couple of weeks, depending on how much of the colony survived. In a brand-new tank, ammonia persists until the tank cycles. Water changes keep it at a safe level in the meantime.

Is ammonia worse in hard water?

Hard, alkaline water usually has a higher pH, and higher pH means more of the ammonia is in its toxic form. Hardness itself isn't the cause. The pH that comes with it is.

Cover photo (cropped): Ben P / iNaturalist, CC BY 4.0