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I found more than one (old) websites (in German) (http://www.teichratgeber.de/wodkafilter_bau01.html and http://www.ohligers.de/Nitratfilter1.htm and there're probably more) which consider a biological nitrate filter a cheap and easy way of denitrification. Other research is difficult because it takes places in non-Q&A forums which are overly painful to read. I found 3 references to the instructions above.

My research concludes that one needs a redox sonde for about 150 €, a dosage pump for another 60 € (and you probably want to spend double for some basic quality) and a potentially complex chain of filter chambers (not expensive, but not free neither) in order to avoid bacteria duff. The danger of the filter producing extremely smelly sulfuric products is omnipresent and makes it difficult to leave the filter unattended during vacation - or requires electronical regulation hardware which will cost a multiple of the basic setup described above.

A anion exchange granulate-based filter for a 600 l aquarium cost 150 € (or 500 € for 2500 l) and requires recharging every x weeks in boiling water for half an hour. The granulate lasts about 10 years; seporax, the filter media in biological filter, will certainly last longer, but some wearout will happen to it over the time, I assume. The filter shouldn't stop running, but the granulate isn't damaged as long as water flows through it, no matter how much nitrate it contains or whether it's charged or not.

I don't see the point. Am I missing something? Did the conditions/prices change over time (the second link estimates the cost at 800 DM for a 700 l aquarium which is ~400€)? Or did I just hit two pages based on superficial research?

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For biological denitrification to take place you need oxygen-free water, as biological denitrification can only take place if there is no oxygen present.

In the gravel/sand of your tank there is an area where the oxygen is depleted due to the bacteria breaking down waste converting ammonia to nitrite (this uses oxygen) and bacteria then use nitrite to produce nitrate (this happens in an oxygen-free environment) and this is the area where denitrification takes place; the resulting nitrogen is absorbed by the water and released to the air. Some extensive information could be found in this article.

All of this is very hard to do inside your filter system; the water flow is simply too fast to have an area without oxygen for the time needed to give the denitrifying bacteria time to do their job.

In a filter system you have the bacteria creating nitrate, growing directly on the substrate and on top of these bacteria grows the bacteria converting ammonia to nitrite (on top the bacteria converts ammonia to nitrite and the bacteria in the layer below convert nitrite to nitrate).

So the simplest way to remove nitrate is to use plants (floating and underwater plants).

I have read your links and I do understand the German text, but not completely.

But I can say the filter mentioned and how it is built have limited possibility to remove nitrates from the water, as it uses oxygenated water to some degree and this makes it uneffective.

What you can do is to build a separate plant filter to remove the nitrates and this filter is very close to free to run after you have made it (the only cost is the lighting and cutting of the plants as they grow). This is what we the people having garden ponds use to get rid of nitrate in the water, but we do not need the extra lighting as it is normaly outside in the sunlight.

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  • Good summary, thank you. I think you're referring to the Deep sand bed method (also possible with acceleration by diffusion aka Berlin method). That only works in saltwater aquariums, right? Does denitrification happen in a noticable way (i.e. mg/l) in a freshwater aquarium as well? Afaik deep sand bed bacteria cannot life when oxygen is present (obligate anaerob), but that doesn't mean that other bacteria - like the one fed in the starch or vodka filter - can't. – Karl Richter Jan 26 '18 at 14:07
  • @KarlRichter in fresh water it is very hard to controll the nitrate to nitrogen prosses and avoid the unwanted prosesses where the production of suphurdioxide happens in the gravel/sand so this is why i think the use of plants are a safer way to remove the nitrate.denitrification in the substrate do happen in fresh water but at a too slow pace to lower it to a safe level,the area of the substrate is simply too low in a aquarium but outside in a natural pond where the water is shallow and the surface is large it will work to reduce the nitrate level. – trond hansen Jan 26 '18 at 15:06
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In a well balanced aquarium you shouldn't need an additional denitrification filter. The filter material used as well as the ground material should provide enough area and living room for denitrifying bacteria. Both work as such filters and are cheap.

Also the filter in your second link is really just a home made aquarium filter and very cheap. You don't need any probe or automated systems.

You basically just need a dense area with lots of surface (such as different ceramic or vulcanic materials) and relatively low water flow (i.e. not too fast). That's it. Most filters offer this in some way or another (typically after or in combination with a mechanical filter).

If you still have a nitrate problem, you should try to do bigger water changes more often or possibly introduce more fast growing plants. It's also possible that you're cleaning your filter too often or too aggressive (e.g. the boiling part would be a big no-no for me). I'm actually not really sure where you got the part with boiling filter material. Doing so you'll kill the bacteria you want to keep.

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  • The denitrification filter I'm referring to in my question are for additional denitrification in situations where daily or permanent water changes are necessary (discus aquariums with few plants). They have their purpose, but I agree that the nitrate can be reduced like you describe in your answer. Regarding the boiling process: This refers to an anion-exchange-based filter, not a biological one, i.e. there's no functional bacteria living in the filter and boiling recharges the filter effect. – Karl Richter Jan 30 '17 at 11:54
  • @KarlRichter That's what I'd call chemical, not biological. Maybe you should add those details to your question to better describe your scenario. – Mario Jan 30 '17 at 22:53

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