How to Lower Ammonia in a Fish Tank: Steps and the Long-Term Fix

Test ammonia, nitrite, pH and temperature, and write the numbers down

Follow the kit’s directions; the instructions for API’s Ammonia Test Kit (API is a test-kit maker) add: “Do not pour test tube contents back into aquarium or pond.” Keep pH and temperature beside the ammonia figure: Aqueon says ammonia “is more toxic at higher temperatures and pH levels above 7.0, and less harmful at lower temperatures and pH levels below 7.0.” Test a glass of untreated tap water too, before any conditioner goes in: UF/IFAS notes that “ammonia may be present in source waters such as some municipal (city) or well waters.” If it reads above zero, ask your water supplier, check that your conditioner’s label names chloramine (the US EPA, a federal agency, says “chlorine and monochloramine must be neutralized or removed if used in aquariums”), and see our guide to what causes high ammonia in a tank, which covers the usual causes.

Stop feeding, or feed much less

UF/IFAS calls this “the first thing to do when ammonia is present”, and Aqueon explains that fed fish “produce more waste”. UF/IFAS adds: “Reducing or stopping feed for a short period of time (e.g., 1–3 days) will not typically have a negative effect on fish (except fry).” If the tank holds fry, ask your fish store or an aquatic veterinarian how far to cut back.

Change part of the water, treated and temperature-matched

Aqueon’s ammonia guide and UF/IFAS both give 25-50%, and UF/IFAS says a change “will help remove some ammonia, assuming the incoming water source does not contain ammonia” (your step 1 tap test). 25% is the one amount inside every range the makers and UF/IFAS give (our reading of their figures; see the table below). Aqueon says to “always treat tap water with a water conditioner to neutralize chlorine and ammonia before adding it to the aquarium”, so use a conditioner whose label names both, and that “replacement water should be the same temperature as the aquarium water.” Aqueon’s don’ts also rule out buckets and other equipment “that have been used with soap or other chemicals”. These are freshwater steps; saltwater keepers prepare new water as their salt mix maker directs.

Find and remove the source

Clear uneaten food, count your fish and remove any dead fish you find. Practical Fishkeeping, a UK fishkeeping magazine, lists “dead fish decomposing in the tank” among the causes to investigate, with overfeeding, overstocking and a filter washed in tap water. In an established tank, API’s FAQ says “it may be necessary to syphon your aquarium gravel to eliminate excess waste and uneaten food particles that have gotten buried.” Do it in part: Seachem, a water-treatment maker, recommends “to use a siphon to clean out about 1/3 of the substrate with every water change (once per week)”, which it says avoids “the ammonia and nitrite spikes that can result from over-enthusiastic gravel cleaning”. For a new tank, Aqueon’s first-60-days guide, in its days 15 to 30 section, says “Don’t vacuum the gravel yet, as you may disrupt the good bacteria that are just starting to colonize your aquarium” and, for the filter, “Do not disturb biological media at this time.”

Protect the filter bacteria

Aqueon explains that beneficial bacteria (nitrifying bacteria) in the filter, the gravel and on surfaces in the tank convert ammonia to nitrite, and it lists “a biofilter that’s too dirty or too clean” among the causes of high ammonia. Practical Fishkeeping’s rule: “Only wash filter media in old tank water, not under the tap, and always replace media bit by bit, not all at the same time.” If your filter takes a single cartridge, follow your filter maker’s directions. Oxygen matters to the bacteria too: UF/IFAS notes that “low levels of dissolved oxygen limit the ability of nitrifying bacteria to convert ammonia and nitrite”, which is a reason to keep the filter and any air pump running. Our guide to cleaning a hang-on-back filter with tank water shows the method.

If you cannot change enough water yet, use an ammonia binder as a bridge

API’s FAQ says its Ammo-Lock “converts poisonous ammonia into a non-toxic form, making your aquarium a safer environment for fish until you can perform a partial water change.” Seachem says its Prime conditioner “may be used during tank cycling to alleviate ammonia/nitrite toxicity”, and that its AmGuard “is ideal for treating emergency situations”. Follow the product’s label for the amount, and keep changing water and testing: these are the makers’ claims for their own products, and UF/IFAS says binders and water changes help “in the short term”. A binder can change what your kit shows, and the sources disagree on which kits. UF/IFAS says binders and many water conditioners “will also cause false high ammonia readings with the Nessler’s method”, while the salicylate method “is not affected by ammonia binding products or formalin treatments.” Seachem, which sells its own free-ammonia kit, says that salicylate and Nessler kits raise the sample’s pH so that “all ammonia removal products will breakdown and rerelease the ammonia, thus giving you a false ammonia reading”, and of its AmGuard that with traditional ammonia test kits “a false positive reading will result.”

Retest more often than usual until ammonia and nitrite both read zero

UF/IFAS says: “Whenever ammonia is found, increase the frequency of testing until the problem is corrected.” The makers’ intervals are for new tanks: Aqueon says to test every two to three days while a tank cycles, and API says to use its kit “several times a week for 4-6 weeks when you start a new aquarium”. Test nitrite each time, because Aqueon notes that “a rise in nitrite usually follows an ammonia spike”; our guide to why nitrite gets high in an aquarium covers that reading. If ammonia keeps climbing, Practical Fishkeeping says to “do additional water changes to keep the levels low.” If you have used a binder and the reading stays above zero, your kit’s method may be the reason (step 6): ask the kit maker or a fish store which method your kit uses, and keep changing water while you find out.