The Wonderful and Mysterious World of Denitrification

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THQ created the topic: The Wonderful and Mysterious World of Denitrification

Please bear with me as I am a beginner fishkeeper delving into more advanced topics that I don't yet fully comprehend and have minimal experience with. From what I have read on nitrate level, and it appears most recommend < 20 for optimum fish health. Apparently there have been referenced studies on even extremely hardy fish like the fathead minnow, which can survive nitrates of 1300+ for a few days before dying, that have shown deleterious effects over time from nitrates as low as in the 20s. Between those 2 extremes, there is a spectrum of physiological adverse effects, from minimally significant to causing severe illness.

Therefore, 0 or at least < 20 ppm is ideal, 24/7.

The ways to reduce nitrates in freshwater tanks are:

Reduce ammonia (thus reducing nitrates)
- reduce feeding
- reduce stocking
- reduce mulm and detritus build up by regular gravel vaccing, and also things like the Eheim recirculating microfilter
- chemical removal eg., purigen, ammo-lock, etc

Denitrification methods:
A. Water changes.
- Dilute only, not eliminate
- Labour intensive and annoying
- Nitrate level gradually builds up, spikes, then is removed by a percentage of the water change. So for some portion of the time, nitrate level may not be ideal.
- can be disruptive and stressful to the fish, especially large ones, and especially if some characteristics of the tank water and new water dont match, thus lowering their immunity and resistance to disease.

The PERFECT solution would be a constant process of nitrate removal, preferably natural.

Please note with all of the below methods some SMALL WATER CHANGES ARE STILL NECESSARY to replace minerals lost and unmeasured toxins, but I believe regular gravel vaccing will exchange enough water for this. Some Walstad tanks have not had water changes for YEARS.

These are the options available:

B. Plants
1. In the main tank
2. In a refugium
- in the sump
- in an overflow
- in a HOB filter

- requires compatible fish
- requires lighting and sometimes CO2
- requires sump/HOB filter, etc

As I am planning an oscar tank with canister, and don't have a sump, plants aren't really an option. The few plants that can grow in a HOB filter are probably insignificant in large fish tanks.

C. Algae
1. In the main tank (undesirable)
2. Algae scrubber, where algae is continuously grown, take up nitrates, and manually shed.
Commercial kits exist eg., hang on glass upflow with LED lighting.
Requires lighting and weekly maintenance to shed algae.

D. Denitrification bacteria (NO3 -> N2 gas)
These require anaerobic conditions so need specialized environments to survive. There are specialized media such as Seachem denitrate which by their design have an anaerobic core for these bacteria to thrive. Flow rate required is very low, < 50 GPH for anaerobic conditions. No canister has such a low flow rate.

1. Deep sand bed (DSB).
Requires 4-8" of fine grained sand, and fish that won't disturb it (not for cichlid tanks), otherwise you may get liberation of poisonous hydrogen sulfide.

2. Nitrate reactor
This looks like a protein skimmer but is full of biopolymers that anaerobic bacteria multiply on. Requires feeding the bacteria a carbon source such as alcohol (or you can even use sugar). Some top of the line models such as Deltecs have fully automated systems that feed this carbon source automatically. AFAIK this is THE most effective way to remove nitrates, achieving virtually 0 ppm 24/7, which water changes can never hope to do. However, there is a decent initial cost (about the same price as a moderate canister filter), as well as ongoings with carbon source, etc.

Sosban Fach did a great write-up on this forum about his experience with his oscar/sev tank and a Deltec nitrate reactor. His main issue was high nitrates in tap water, but the principles are the same. 100 ppm in, 0 ppm out of the filter.

E. Chemical removal
This can be of nitrate itself or of ammonia before it enters the nitrogen cycle, thus indirectly reducing nitrates.
- adsorption filters
- ion exchange resins
eg., nitra-zorb

From what I have read, these are not as effective one would think, and quite expensive to maintain/replace. Recharging them takes just as much work as doing more water changes.

In conclusion, for an oscar tank (no plants, no DSB), especially without a sump (no refugium), my choices are limited. The most effective way seems to be the virtually maintenance free nitrate reactor in order to drastically reduce BUT NOT ELIMINATE the need for water changes (would still be done during gravel vaccing debris). This will make me happier, as I do far less water changes, yet also make my fish happier, as they have a consistently, virtually nitrate free environment that reflects their wild origins eg., Amazon river 1.2 ppm nitrate, Orinoco river system even lower.

It's really a win-win situation. Apart from the initial setup cost, is there any downside to this? What are people's thoughts?


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Rocksor replied the topic: The Wonderful and Mysterious World of Denitrification

For the nitrate reactor, you need to have the water that leaves it to be mechanically filtered prior to entering the aquarium because of the bacterial slime by product of the reactor. The mechanical filtration (filter floss or equivalent) needs to be changed everyday, otherwise the slime will turn back into nitrate.

Other folks use an automated drip system to change 25%-50% of the water everyday.
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THQ replied the topic: The Wonderful and Mysterious World of Denitrification

Hmmm I did not realise the mechanical filtration had to be changed every day. Maybe not worth the trouble then.

An automated drip system is ideal, but unfortunately I just don't have the right room/house setup to make that practical.

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