How to Lower Nitrites in a Fish Tank, Step by Step

Test nitrite and ammonia, and write both numbers down

Follow your kit’s directions; step 8 repeats the test.

Feed less until the reading falls

Fritz’s first corrective action is: “Temporarily reduce feeding if any nitrite or ammonia is detected.”

Change part of the water, treated and temperature-matched

First test a glass of untreated tap water with the same kit. If it reads nitrite above zero, a water change can lower a higher tank reading only down toward the tap’s number (worked out, not sourced): ask your water supplier, and see the tap-water part of our nitrite causes guide. Aqueon says to “always treat tap water with a water conditioner to neutralize chlorine and ammonia before adding it to the aquarium”, and “Replacement water should be the same temperature as the aquarium water.” Use a bucket and siphon that have never touched soap: Aqueon’s don’ts include equipment “that have been used with soap or other chemicals”. For how much, see the table below. These are freshwater steps; saltwater keepers prepare new water as their salt mix maker directs.

Clear waste from the bottom, and go easy on the gravel

Fritz says to “remove as much waste from the bottom of the aquarium as possible”. For the gravel itself, work 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”. In days 15 to 30 of a new tank, skip the gravel: Aqueon’s first-60-days guide says “Don’t vacuum the gravel yet, as you may disrupt the good bacteria that are just starting to colonize your aquarium.” Aqueon’s guide gives no gravel rule for other weeks of a new tank; ask your fish store before vacuuming gravel in a tank under 60 days old.

Protect the filter bacteria and add air

Do not swap cartridges or rinse media under the tap now: Practical Fishkeeping, a UK fishkeeping magazine, notes that media “washed in tapwater or replaced with new” holds few bacteria. If the filter clogs, its rule is: “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.” In days 15 to 30 of a new tank, Aqueon says to rinse solid waste off cartridges as needed (in old tank water, per Practical Fishkeeping’s rule) and “Do not disturb biological media at this time.” That rinse covers mechanical media only. Our guide to cleaning a hang-on-back filter with tank water shows the method. Keep the filter running and add an airstone if the tank has none, since UF/IFAS notes that “low levels of dissolved oxygen limit the ability of nitrifying bacteria to convert ammonia and nitrite”.

Add nitrifying bacteria, or seed from a healthy tank

Aqueon explains that “a different set of nitrifying bacteria” turns nitrite into nitrate. Bottled bacteria are sold for this: Seachem, for example, says its Stability is designed to “rapidly and safely establish the aquarium biofilter” (the maker’s claim; follow the label). For a new filter, Practical Fishkeeping suggests you “transfer some media from an existing filter”, and Aqueon specifies media “from an established, healthy aquarium”.

Consider a maker’s bridge product, and follow its label

These are makers’ claims for their own products; keep changing water and testing while you use one. Seachem says its Prime conditioner “may be used during tank cycling to alleviate ammonia/nitrite toxicity” and contains a binder that renders ammonia, nitrite and nitrate non-toxic, “allowing the biofilter to more efficiently remove them”; a binder or conditioner can change what an ammonia kit shows (step 8). For freshwater tanks, API says aquarium salt “reduces the harmful effects of nitrite on your fish”, with two cautions: “Aquarium salt does not evaporate and is not filtered out, so it should only be added with each water change.” and “Care should be taken when using salt with live plants, as some plants may be sensitive.” The salt section below has the rest.

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

API’s interval is for a new set-up: it says water “should be tested for nitrite every other day when your aquarium or pond is first set up”. For a spike in any tank, Practical Fishkeeping says: “Test daily and be prepared to make further water changes to reduce the levels.” A conditioner or binder can change the ammonia reading, 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 “Other kits (salicylate or Nessler based) determine the total ammonia by raising the pH of the test solution to 12 or greater”, so that “all ammonia removal products will breakdown and rerelease the ammonia, thus giving you a false ammonia reading.” If ammonia stays above zero after you used one, ask the kit maker or a fish store which method your kit uses, and keep changing water while you find out. Our guide to why aquarium ammonia gets high covers that reading. Once both read zero, API’s kit page moves to testing “once a week after the biological filter has been established (in about 4-6 weeks)”; until then, in a new tank, keep to the intervals above.