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I have a new tank which I didn't cycle before adding fish (didn't know about it).

However since then I have read about cycling and have purchased a fresh water test kit. I tested my water and got the following:

  • Ammonia 0
  • Nitrite 5 ppm (this is the highest lvl my color chart has)
  • Nitrate 0

Its been like that for a week despite 30% water changes every two days.

Everything I have read on internet forums say my fish should be dead. However they seem fine. They are not gasping for breath at the top of my tank like would be expected for nitrite poisoning.

I have 6 tetras and a bristlenose pleco in a 60 litre tank.

Has nitrite toxicity to fish been overstated? Why are my fish still alive?

Update Its been two weeks now, Nitrites are still at 5 ppm, I have upped the water changes to 50% every day, with no effects on the Nitrite readings. I still haven't lost any fish. My guess is that the nitrites are actually well over 5 ppm so the 50% water changes are not enough to bring it below 5 ppm. I'm starting to see Nitrates in the tank so the cycle is progressing.

Updates 2 Its been three weeks and the nitrites have now gone to zero. I havn't had lost any fish, or seen any signs of nitrite poisoning.

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  • How did you test it? With those strips that you have to dip in water, or a liquid test kit? Those strips can be very inaccurate.
    – Diether
    Sep 23, 2016 at 7:03
  • I used a liquid test kit from API: aquariumsuppliesnz.co.nz/products/…
    – trampster
    Sep 23, 2016 at 8:44
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    I did some research on API test kits and found a statement from them saying that the nitrate can read low because of solidification in the bottle, but a sharp tap or vigorous shaking will fix that. They also said they have never seen a case of the nitrite over reporting. The only other complaint I have seen is that the 5 mill mark is not that accurate but mine seem fine. Most people who have done comparisons with other test kits say the API kits give the same readings as other test kits. I have therefore no reason to believe that the test kit is over reporting my nitrite levels.
    – trampster
    Sep 24, 2016 at 0:35
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    How about trying the opposite way? Have you tried testing some fresh water or table water? Maybe even distilled water? Do they show the same readings?
    – Mario
    Sep 24, 2016 at 6:28
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    I have tested the tap water it shows 0 PPM nitrites. I don't think there is anything wrong with the test kit.
    – trampster
    Sep 24, 2016 at 7:57

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www.theaquariumwiki.com has a good answer.

According to it Nitrites at 10-20 ppm is considered lethal. (Although it does vary by species)

At 5 ppm the nitrites are not high enough to be lethal to my fish.

However those levels could still be doing damage to my fish. Including damage to the gills and long term damage to their immune system as well as stress. Any detectable level of nitrite could be doing damage.

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