Knowing how to brew your own stock solutions give you great control on what gets in your tank and makes you save lot of money. Unfortunately, after two years without a freshwater tank, I forgot how to do this. I searched the web looking for answers but all I could find is a few calculators. Online calculators are great tools but who knows how accurate they are? Anyway, I don’t want a calculator to tell what the answer is; I want to know how to find it myself.

I found pieces of information here and there and decided to write this short summary of what I’ve learn. I hope, this will help those who want to give that a try.
I asked a few folks from Futura Science Forum to have a look at my calculations. They told me that was ok but I would appreciate if someone could double check everything again and confirm the accuracy of my calculations.

Some Conversion Factors before to start

1) 1ppm = 1mg/L (one part per million = one milligram per liter)
2) 1 miligram = 1/1,000 of a gram (1,000 milligrams = 1 gram)

How to make a fertilizer solution for the planted aquarium

I am not sure how to explain this in a short answer so what I’ll do is to walk you through the staps of making a Potassium Nitrate (KNO3) solution.
KNO3 will increase both No3 and K levels but to keep things easy, we will forget about K for today.

Step 1
First, I need to find the mass (in percent) of each element in KNO3. Here it is:

Step 2
I now know there is only 13.854% of N in KNO3. Now I need to count how many grams of KNO3 is needed to make 1 gram of N. The formula is the following:

100 X (1 ÷ mass of the element) = how much I need (in gram) to have 1 gram of the element

Which make: 100 X (1 ÷ 13.85) = 7.218 gram of KNO3 to have 1 gram of N

Step 3
Now let’s mix that 7.218 gram of KNO3 with water to make a solution and see what we get.
Since 1 liter of water weight 1000 gram (1,000,000 milligrams), if I pour in 1 gram (1000 mg) of N I will get:

1 gram of N in 1 liter of water.
Or
1 gram of N in 1,000 gram of water (1 liter = 1000 gram)
Or
1000 milligram of N in 1,000,000 milligram of water .
Or
1000 ppm of N (1 ppm = 1 part in 1,000,000 parts)

Step 4
Now, let’s see how much KNO3 I need to increase N by 1 mg in one liter (1 ppm). I use the following formula:

For those who are unsure about how to use this formula, here it is, presented differently:
(1 X 7.218) ÷ 1000 = how much gram of KNO3 is needed to make 1 mg/l of no3 in one liter of water.

The answer: 0.007218 gram of KNO3 in one liter = 1mg/L (1 ppm) of N.

Have you noticed something?? We’re almost back to step 2!
If I wouldn’t have multiplied by 100 in the formula (see step 2), I would have found how much KNO3 I need to increase N by 1mg in one liter!

(1 ÷ 13.85) = 0.07218 gram of KNO3 to make 1 mg/L (1ppm) of N
Instead of
100 X (1 ÷ 13.85) = 7.218 gram of KNO3 to have 1 gram of N

I could have skipped a few step but I wanted to take the time to explain everything and make sure you understand the process. From now end, you can forget about step 3 and 4. Just remember not to multiply by 100 in step 2.

Step 5
I know how much KNO3 is needed to add 1 mg of N in one liter (1 ppm). Now, all I have to do is to multiply that amount by the amount of liter in my tank.

I have a 100 liter tank so I will need 100 X 0.07218 = 0.7218 gram of KNO3 to rise N by 1 mg/L (1 ppm)

Step 6
There is no way I am going to weigh 0.7218 gram of KNO3 for each mg/L (ppm) I need. This is why I need to make a solution with more KNO3 in it.

I dose 10 mg/L (ppm) of N per week in my 100 liter tank. Since I need 0.7218 gram of KNO3 to get 1 mg/L of N in 100 liter, 10 X 0.7218 = 7.218 gram of KNO3 for a week.

If I mix the amount of KNO3 needed for a week with 10 ml of water, we could say that 10 ml of my solution add 10 mg/L (ppm) of N in 100 liters. If I want enough solution for one month all I have to do is to multiply the amount of KNO3 AND the amount of water by 4. Then, I would get a 40 ml KNO3 solution which I can use at a ratio of 10 ml for every 10 mg/L (ppm) I want to add to my 100 gallon tank.

At this point, this is up to you to decide how you want it to be; more or less concentrated.

I tried to make things easy to understand but, when I do, sometime I make things more complicated. I hope this isn’t the case today. If there is something you don’t understant, please use the comment box bellow so I can help you and/or edit that post to make it easier to follow.


Elements in common aquarium fertilizers
Here are details about some of the most common aquarium fertilizers.

Calculating fertilizer solutions for the planted tank5.051

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