How to lower ammonia in an aquarium

AquaLog guide · Published 6 October 2026

What should you do right now?

1. Confirm the reading. Test again, and test your tap water too. Some water supplies are disinfected with chloramine, which is made with ammonia.
2. Change 25–50% of the water with dechlorinator added and the temperature matched. The Merck Veterinary Manual advises changing at least half the water when total ammonia reaches 2 ppm or more.
3. Stop feeding for a day or two, then feed sparingly until ammonia reads zero. Digested fish food is where most ammonia starts.
4. Remove anything rotting: dead fish or snails, uneaten food, decaying plant leaves.
5. Add aeration. Ammonia damages gills, so give the fish as much oxygen as you can.
6. Leave the filter alone. Don't replace or scrub the media; it holds the bacteria that remove ammonia.
7. Test again the next day, and repeat the water change while ammonia stays above zero.

Water conditioners sold as ammonia binders can relieve ammonia poisoning in the short term, according to University of Florida (UF/IFAS) specialists. They buy time; they don't remove the source. Follow the product label.

How dangerous is your reading?

Most test kits measure total ammonia: two forms added together. Un-ionised ammonia (NH3) is roughly 100 times more toxic to fish than ammonium (NH4+), and the share that is NH3 rises with pH and temperature. So the same kit reading is far more dangerous in hard, alkaline water than in soft, acidic water.

These are the shares of total ammonia in the toxic NH3 form, from the UF/IFAS conversion table, which is calculated from the standard equations of Emerson and colleagues (1975). At pH 8 there is about ten times as much toxic ammonia as at pH 7 for the same reading. UF/IFAS warns that tissue damage can begin once NH3 passes 0.05 mg/L (0.05 ppm).

Two rules follow. Treat any ammonia reading above zero in an established tank as a problem, whatever your pH. And don't raise pH while ammonia is present: doing so turns more of the ammonia already in the water into the toxic form. Our guide to aquarium water parameters covers pH in more detail.

How much does a water change remove?

A water change removes the same share of ammonia as the share of water you replace, as long as your tap water has none. A 50% change halves the reading: 2 ppm becomes 1 ppm, then 0.5 ppm after a second change. The fish keep producing more in between, which is why you test again the next day. Use the aquarium volume calculator to work out how many litres 25% or 50% of your tank is.

If the tank has gone weeks without a water change, its chemistry may have drifted far from your tap water. Several smaller changes over a few days are gentler than one huge one; see how often to change aquarium water.

Where is the ammonia coming from?

• A new tank. The filter bacteria haven't grown yet. This is new tank syndrome, and the lasting fix is finishing the nitrogen cycle.
• Too many fish, or too many added at once. The bacteria grow to match the waste they receive and need time to catch up.
• Overfeeding. Most ammonia comes from fish digesting their food, and uneaten food rots into more.
• Something dead. Check behind decor and under plants for a fish or snail.
• A damaged filter. Media rinsed under the tap, replaced all at once, or a filter left off for hours loses its bacteria.
• Medicines. Some, especially antibiotics, can harm filter bacteria.
• Tap water. Supplies treated with chloramine can bring ammonia in with every water change.

Why does ammonia keep coming back?

Water changes dilute ammonia; they don't stop it being made. Fish release it all the time through their gills, so it returns until the filter bacteria can keep up with the waste. That takes weeks, not days: UF/IFAS puts a new biofilter at six to eight weeks, in line with the four to eight weeks a fishless cycle usually takes.

Big, messy fish in a small tank may never reach that balance. Our care guide lists ammonia burn in small tanks among the common problems of fancy goldfish, which need at least 150 litres (40 US gal). The common pleco grows to 45 cm (18 in), needs at least 700 litres (185 US gal), and is listed with poor water quality from heavy waste. If your fish have outgrown the tank, more water changes only delay the problem.

How should you test?

• Use a liquid test kit. Strips are quicker but less precise.
• Test at the same point in your routine, ideally just before a water change, so readings compare.
• Test your tap water once, so you know whether it brings ammonia in.
• Test pH and temperature alongside ammonia; you need both to judge the risk.
• Check the kit's expiry date. Old reagents give false readings.
• If you have used an ammonia-binding conditioner, kits that use the Nessler method can read falsely high; salicylate kits are not affected, according to UF/IFAS.

Daily tests are easier to judge as a line than as a list. Log your readings in AquaLog and you can see whether ammonia is falling between water changes, and when the filter starts to keep up.