[Updated 16/06/2022]
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Have you ever wondered how much ammonia is actually produced in your tank from your feeding? Well turns out - an estimation is pretty easy to arrive at - just needs a few specific bits of information:
How much are you feeding (daily), in milligrams.
The protein content of the food.
The amount of water in your aquarium (or failing that, your aquarium size).
Let's break it all down, using an example.
First - let's figure out how much one is feeding in milligrams. Often we think of the food as 'number of pellets' for example, so let's try to convert that over.
So, I am currently feeding my fish this. If the link is inaccessible or no longer available, well it's just very small pellets. The way to figure it out is to get a scale that can go to very low numbers. I got one from Amazon for like $10 that can measure down to 0.01 gram (10 milligram). So what I did was take put enough pellets onto the scale until it hits 0.1 gram. This is just to make it more accurate. This took 140 pellets. So I know 140 pellets is 0.1 gram, 14 pellets is 0.01 gram, and 1 pellet is 0.0007 gram, or 7 milligrams.
However, I feed my fish about 70 pellets each time, 4 times a day. That is 280 pellets in total. Since 140 pellets is 0.1 gram, 280 pellets is 0.2 gram, or 200 milligrams. Tada!
Your calculations will be different, most likely. Because my pellets are tiny, obviously they each individually weight less. You may only feed 7 pellets each time, and the weight may be the same as my 70 pellets. Anyways.
Second! The protein content. The % of protein in my food is 59%. Unfortunately this information is absolutely needed, otherwise the calculation cannot work. The % can often be found on the product or its website, but not always. If you cannot find out this info, unfortunately you are out of luck.
So why do we care about the protein content and not other components of the food? Well, because a major source of nitrogen is the breakdown of proteins. Certainly other components may also contain nitrogen, but 1. not as significantly, and 2. this is just an estimate, not an exact specific calculation.
Third and last, water volume. I have a ten gallon tank, but the actual amount of water is probably closer to 8 gallons, i.e. around 30 litres, i.e. take away decorations, equipments, substrate, etc. We can of course use the whole tank volume, i.e. 10 gallon. It is an estimation after all. If you do want to account somewhat for things that is taking up tank space to get an estimation of true water volume, just multiply your tank volume by 80%. That should give a good conservative estimate.
Alright, so actual calculations!
f x p x 0.16 x 18 / 14 / w
f = food (mg)
p = (crude) protein content (%)
w = water volume (litres)
Where are the numbers from?
0.16 = average weight of nitrogen in protein
18 = molecular weight of ammonium ion
14 = molecular weight of single nitrogen atom
Let's break down the calculation then.
We get how much of the food is the protein (f x p).
We then get how much of that protein is nitrogen (f x p x 0.16).
We calculate the weight of the ammonium ion produced from said nitrogen (f x p x 0.16 x 18 / 14).
Finally we calculate the concentration of ammonia (ammonium) in mg/l, i.e. ppm (f x p x 0.16 x 18 / 14 / w).
So for me, I have:
f = 200 milligram
p = 59%, or 0.59 (decimals)
w = 30 litres
Thus the calculation is 200 x 0.59 x 0.16 x 18 / 14 / 30 = 0.81ppm, approximately.
Just from feeding alone, I add 0.81ppm ammonia into the tank each day. To be fair, I also am feeding A LOT, hence this is expected.
This is also why I recommend when cycling, to get the tank to be able to handle at least 1ppm ammonia a day for a 'normally stocked tank', and 2ppm for a heavily stocked, heavily fed tank.
A reminder that this is only an estimation of the amount of ammonia potentially produced, and also assuming 100% of the nitrogen source is excreted as ammonia. Of course for growing fish, they will consume some of the nitrogen for their growth. For a tank where all the fish are not really growing anymore, and there are no baby fish or whatever, then it would be expected that more nitrogen is excreted.