After installing an aquarium with:

  • a new filter without bacteria in the media
  • new/washed gravel
  • plants which don't have sufficient roots yet (to absorb nitrate / NO3-)[1]

the nitrite peak occurs. Which one(s) of these conditions (and possible others) cause the nitrite (NO2-) peak? Which one(s) have more or less influence?

I want to evaulate the possibility to install an undergravel filter in a running aqaurium where I'd have to move the gravel and replant plants step by step. I have a 60 l freshwater aquarium.

[1] Citation needed for the fact that plants absorb nitrate with roots.

  • 1
    Oh and regarding the nitrate absorption by roots - I recall having read that aquatic plants use their roots only to anchor them in place, and they are absorbing water and minerals via their whole surface. I couldn't link a reference though, apologies, because I've read that long ago and it was in a book or a magazine, I don't really remember. But I guess it makes sense - terrestrial plants absorb only via roots because only the roots are in the water-soaked soil - on the other hand, aquatic plants are submerged as a whole. And also some aquatic plants don't even have roots.
    – lila
    May 28, 2020 at 11:29

1 Answer 1


The peak occurs because there are not sufficient bacteria to counter the effect. Once the peak starts, the bacteria start to form and break it back down.

These bacteria form in your gravel, your filter, anywhere. If you change the gravel completely you will naturally lose some of your bacteria, but if you keep your filter intact then you shouldn't worry all that much. Keep an eye on the levels and do water changes if it gets to dangerous levels.

Make sure you always leave plenty of time between changing the gravel and filter media so the bacteria have time to form in the new gravel / filter before you take the only ones you have left back out.

To quote from this page:

During the cycling process, ammonia levels will go up and then suddenly plummet as the nitrite-forming bacteria take hold. Because nitrate-forming bacteria don't even begin to appear until nitrite is present in significant quantities, nitrite levels skyrocket (as the built-up ammonia is converted), continuing to rise as the continually-produced ammonia is converted to nitrite. Once the nitrate-forming bacteria take hold, nitrite levels fall, nitrate levels rise, and the tank is fully cycled.

If the established tank uses an undergravel filter, nitrifying bacteria will be attached to the gravel. Take some of the gravel (a cup or more) and hang it in a mesh bag in your filter (if you can), or lay it over the top of the gravel in the new tank (if it has an UGF).

  • 1
    I already performed the installation of the undergravel filter and figured that a well running external filter should be able to handle the impact of removing the gravel. Measuring every day I didn't see any increase of nitrite, a slight, but non-dangerous increase of nitrate (probaly because of the destroyed roots of plants) and a slight increase of ammonium. Your answer is valuable as reference and might encourage others to install an (extra) undergravel filter. It's awesome :) Thank you. Oct 4, 2016 at 23:35

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