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So I’ve recently set up a small 5 gallon tank, planted heavily, and started a fishless cycle.

I got stuck on my nitrite stage as a result of overdosing ammonia, but it finally came back down 3 days ago.

As recommended to me, I did a big water change and then dosed ammonia back up to 2.0 ppm to ensure that it would be processed down to nitrates in 24 hours. My little ammonia eaters have been doing their job really well (back to zero in no time), but much to my surprise my nitrites are once again stuck somewhere between 2.0 and 5.0 ppm.

What’s going on? I was so sure that my nitrite eaters were finally ready to go, but now they seem to be slacking and/or disappearing.

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  • I am glad I started keeping fish so many decades ago ; you did not need to test anything. You put water in an aquarium , let it set a few days , with plants if you had them . Then put in fish- no problems. I did buy a thermometer before long. Feb 23 '19 at 22:16
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The main problem is that the bacteria that convert nitrite to nitrate do multiply very slowly, and this is the reason for why the cycling of a tank takes a long time.

Here is a Wikipedia article about Nitrobacter.

Nitrobacter do multiply by growing and dividing itself and this takes about 24 hours, so one single bacteria divides itself in two in 24 hours and four in 48 hours and so on.

The bacteria Nitrosomonas that convert ammonia to nitrite do multiply by shooting buds of themselves and this takes a very short time, so they can double their number about every two hours or so.

Here is a Wikipedia article about Nitrosomonas.

Both types of bacteria are equally important, so you need to be patient; your tank will reach biological balance, just give it time.

There are different species of both Nitrobacter and Nitrosomonas around the world and each of them have differences in growth rate and how effective they are and in their needs of nutrients, so my answer must be seen in the light of this it is a general answer for the types of bacteria.

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    Interesting answer. The page you linked for Nitrobacter says the doubling rate is 13 hours, not 24 ? And the page of Nitrosomonas states that cell division may take several days, so what's the truth ?
    – Manuki
    Feb 25 '19 at 16:17
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    the reason nitrosomonas can take a long time to divide and multiply is because they need a lot of ammonia for energy to start multiplying but when they do have ammonia they multiply fast but they are photophobic so light might slow them down again:) .for nitrobacter to start growing nitrite have to be present and they use a lot of oxygen too. Feb 25 '19 at 16:58
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    continued:the prosses of creating nitrate from nitrite is an energy intensive reaction this makes them multiply at a slower pace. Feb 25 '19 at 17:09
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    Thanks for the precision. And I would add for anyone wondering, that the oxygen comes from surface movement made by the filter. (that and water changes, but then you also change portion of your bacteria)
    – Manuki
    Feb 25 '19 at 17:12

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