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safe levels of ammonia, nitrite and nitrate
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Joined: 18 Mar 2006

PostPosted: 2007.01.03(Wed)11:18    Post subject: safe levels of ammonia, nitrite and nitrate Reply with quote

Hi guys!

I have been keeping fish for about four years and I'm not new to the nitrogen cycle, I understand how it works, but I cannot answer all the questions my sister has asked me for her A level chemistry project.

I do not add fish to my tank until ammonia and nitrite are zero, but she would like to know what safe levels of ammonia, nitrite and nitrate are before it becomes harmful to fish?

Also what effects do high levels of these have on fish, I know that it stresses them and can cause stunting, is there anything else it can do?

Thanks in adavance!

Best Regards.
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Joined: 13 Oct 2003

PostPosted: 2007.01.03(Wed)14:35    Post subject: Reply with quote

Here are the safe levels for fish

Ammonia 0ppm but fish can handle .5-1 ppm for a short duration
Nitrite 0ppm but can handle 1.5 ppm or so for short periods
Nitrates 40 and under is alright but can handle 60-80 range for short periods.

With all of them they will stress the fish but with Ammonia it will actually poison the fish and kill it.
# Symptoms: Fish gasp for breath at the water surface
# Purple or red gills
# Fish is lethargic
# Loss of appetite
# Fish lays at the bottom of the tank
# Red streaking on the fins or body

Here are the Nitrite symptoms
# Symptoms: Fish gasp for breath at the water surface
# Fish hang near water outlets
# Fish is listless
# Tan or brown gills
# Rapid gill movement
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Joined: 18 Mar 2006

PostPosted: 2007.01.04(Thu)4:39    Post subject: Reply with quote

Thanks a lot!

I have one last question for you, could I possibly have your fore and surname? It seems odd, but wiht your permission, my sister would like to give you a source for her information.

My sister says many thanks, that was the exact information she was looking for.
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Joined: 16 Jun 2006
Location: UK

PostPosted: 2007.01.04(Thu)5:16    Post subject: Reply with quote

Sorry to but in.

I know this question has been asked for a theoretical study, but lots of people read public forums, who may well see the answer, and take the answer out of context.

You also get the ones who will seach from board to board for the easiest route, who will turn a perfectly sensible answer to a theoretical question around, and say "Well so and so says this amount of Ammonia is safe".

This is why the only real answer is;

The only safe levels are 0.

What is a short period, what would happen if a fish, weakened by Ammonia poisoning then comes up against Nitrite poisoning a few weeks later. So many other parameters need to be taken into account.

Sorry to be a pedant.

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Joined: 29 Mar 2005
Location: Melbourne, Australia

PostPosted: 2007.01.04(Thu)5:40    Post subject: Reply with quote

Just to add a bit onto the topic (dammit! I see Bob beat me to it when I hit preview)...


There was a EPA study done on ammonia toxicity, you will find that the LD50 (50% casualties) reading was about 0.7ppm over 4 days, while 1 hour lethal dose averaged out at about 7ppm. The non-lethal concentrations will vary a lot (particularly in the latter 1h LD50 dose) between different pH, temperature and salinity readings as these affect the equilibrium between NH4 and NH4+ (acidic, hot, saline conditions increase toxicity). Note that 'free ammonia' (NH3-N) is very toxic, while 'ionized-ammonia' (NH4+-N) is less so. Individuality (individual fish from the same species as well as different species comparatively) also affects the lethal dose.

Toxicity is caused by rising blood levels of ammonia due to external ammonia concentrations slowing down excretion of NH3--N (reduced ammonia-nitrogen - I think thats the right chemical, someone correct me if I'm wrong). This results in NH3+ interfering with regulation of K+ (both + ions), which is important for many cellular processes and ends in convulsions and death. Ammonia also has cytolytic effects, and this is particularly important in the kidney (damage to blood vessels), gills (damage to cells resulting in less oxygen diffusion) and liver (possible failure when concentrations are high enough - this is the primary site of detoxification).

As responsible fish-keepers, the only safe level of ammonia in our fish tanks is 0ppm. Any more can result in long-term damage that is not immediately visible, or death.


As with ammonia, nitrite is affected by individual fish, salinity (to be specific, increasing Cl- ion concentration reduces toxicity) and temperature (which increases toxicity)...there has to be other factors but my memory is a little fuzzy right now.

Reference (sorry you require some sort of academic access, PM me if you are interested in the paper): Alcaraz, G. & Espina, S. (1995) Acute toxicity of nitrite in juvenile grass carp modified by weight and temperature. Bulletin of Environmental Contamination and Toxicology 55, 473-478.

Here is something I wrote a while back on the topic when I had some time (yes, I'm too lazy to write it again so I just copied it)'s stalled at the moment due to time constraints:

unissuh wrote:

Nitrite poisoning
The classic symptoms of nitrite (NO2-) poisoning or brown blood disease are gasping at the surface, listlessness, tan/brown gills, rapid gill movement and hanging near filter or powerhead outlets. In short, experiencing oxygen deprivation.

How does this work? Nitrite is uptaken by the gills, and binds to haemoglobin molecules resulting in a molecule called methomoglobin. Unfortunately, unlike haemoglobin, methomoglobin is not capable of carrying oxygen and accumulation of this molecule ultimately results in asphyxiation.

Cl- can reduce nitrite toxicity
Fortunately for us aquarists, salt (or more specifically the Cl- ion) can reduce nitrite toxicity in fish. Nitrite has to be uptaken to cause poisoning, and is competitively inhibited by Cl- ions (Williams and Eddy, 1986). In fact, increasing the ambient Cl- concentration is possibly the single most important action in preventing nitrite uptake and toxicity (Bath & Eddy, 1980; Perrone & Meade, 1977). Note that this Cl- ion does not have to come from salt, other compounds like Calcium chloride (CaCl2) can be just as effectively used.

A more recent paper also explores the varying sensitivities of different species relative to Cl- and nitrite uptake (Tomasso & Grosell, 2005).

How much salt?
A 6:1 Cl-:NO2- ratio should be maintained to effectively prevent nitrite uptake (Eddy et al, 1983); if you have 1ppm of NO2- simply make sure that you have 6ppm of Cl- present in the water.

Lets assume 1tsp of salt weighs 5g (in reality it would weigh 6g to 6.5g on average).

The molecular weight of NaCl is 58.44gmol-1. The molecular weight of Cl- is 35.45gmol-1. Therefore the percentage of Cl- in NaCl is 60.66%.

60.66% of 5g is 3.03g of Cl- in 1tsp or 5g of salt.

6ppm can also be expressed as 6mg/L or 0.006g/L. Therefore adding 3.03g of Cl- to 505L will give you a concentration of 6ppm Cl-.

505L works out to approximately 130gal.

So, the addition of 1tsp or 5g salt will protect against 1ppm of NO2- in 500L or 130gal of water.

Bath, R. N. & Eddy, F. B. (1980). Transport of nitrite across fish gills. J Exp Zool 214, 119-121.
Eddy, F. B., Kunzlik, P. A. & Bath, R. N. (1983). Uptake and loss of nitrite from the blood of rainbow trout, Salmo gairdneri Richardson, and Atlantic salmon, Salmo salar L. in fresh water and in dilute sea water. J Fish Biol 23, 105-116.
Perrone, S. J. & Meade, T. L. (1977). Protective effect of chloride on nitrite toxicity to coho salmon (Oncorhynchus kisutch). J Fish Res Board Can 34, 486-492.
Tomasso, J. R. & Grosell, M. (2005). Physiological basis for large differences in resistance to nitrite among freshwater and freshwater-acclimated euryhaline fishes. Environ Sci Technol 39, 98-102.
Williams, E. M. & Eddy, F. B. (1986). Chloride uptake in freshwater teleosts and its relationship to nitrite uptake and toxicity. J Comp Physiol B 156, 867-872.

Again, as with ammonia, as responsible fish-keepers, we should endeavour to keep nitrite at 0ppm. It is less toxic than ammonia, and there has to be a concentration at which it is non-harmful (toxicity only results from low oxygen uptake or high concentrations being cytolytic as far as I know), but I have not found a paper that has done this sort of long-term study (if you think about it, every single species will be different as well as a massive sample size required as biological variation between individuals is huge...not to mention other factors).

EDIT: Sorry, can't offer much on nitrate - I have not really looked into the topic much at all. Just the typical "under 40ppm is OK for typical species, under 20 for sensitive species"...

EDIT2: Okay, I just looked at what I wrote and it's another bloody essay! If this has gone way over your head, don't worry...I have a penchant for confusing people.
Fishing in the Rivers of Light

Last edited by unissuh on 2007.01.04(Thu)8:06; edited 4 times in total
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Joined: 16 Jun 2006
Location: UK

PostPosted: 2007.01.04(Thu)5:46    Post subject: Reply with quote

You taking up pedantry as well unissuh Wink

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