What is CN Ratio in BIOFloc Technology and Why its important - Road to BIOFloc

What is Carbon to Nitrogen Ratio i.e. CN Ratio in BIOFloc

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Carbon to Nitrogen Ratio (CN Ratio) is a process of controlling the amount of Nitrogen in water. Most popular CN ratios are 10:1 and 15:1, which means in CN Ratio 10:1, you need 10 Carbon Sources to kill 1 Nitrogen. In aquaculture, CN Ratio is calculated based on Protein Percentage of Feed and Total Ammonia Nitrogen (TAN)

CN Ratio with Zero Water Exchange

The CN Ratio Calculation is based on Fish Feed Protein Percentage and the below chart shows the CN Ratio of the feed materials.

Protein Percentage (%) CN Ratio

15 21.5
20 16.1
25 12.9
30 10.8
35 9.2
40 8.1

If you are using 30% protein feed then according to the above chart the CN Ratio of 10:1 is fulfilling. As per the studies, the CN Ratio of 10:1 and 15:1 are more successful. It has been proved the CN Ratio of 10:1 does not require even water exchange.

In a BIOFloc system, it’s recommended to feed low protein feeds. You will have better control over Ammonia and the mortality rate will be less.

Here is an example;

Let’s say the tank contents 100kg of fish biomass and you are feeding 4% of the bodyweight which equals to 4kg feed (4000 grams of feed)

The feed is made up of with Carbohydrates Proteins

Let’s calculate Carbohydrates in feed;

4000 grams of feed contents 90% of dry matter i.e. 3600 gram and 10% moisture i.e. 400 grams on it

Out of 90% of dry matter i.e. 3600 grams, 70% get wasted i.e. 2520 gram and only 30% get digested by fish

For C:N Ratio calculation, you need to consider 70% waste i.e. 2520 gram, because this 70 % of waste is responsible for creating ammonia inside the tank. Don’t calculate CN ratio on the basis of total feed given to fish i.e. 4000 grams.

It’s known that feed contents 50% of carbon in it. So, 1260 grams of Carbon can be found on 70% of waste feed i.e. 2520 grams.

So every day you are adding 1260 grams of Carbon into the water.

The feed that we are using here contents 30% of protein i.e. 30% of 2520 = 756 gram

In 756 gram of protein, Nitrogen contents 16% = 120.96 gram (N)

If you are using BIOFloc in a proper and scientific way then you no need to add Carbon into the tank.

If you look at the C:N Ratio value its 1260:120.96 which is 10:1 ratio.

C:N Ratio 10:1 = 120.96 x 10 = 1209.6 gram

We already have Carbon of 1260 gram = 1209.6 – 1260= (-50.4) gram – Zero Carbon needed

C:N Ratio 12:1 = 120.96 x 12 = 1451.52 gram

We already have Carbon of 1260 gram = 1451.52 – 1260=191.52 gram Carbon needed

C:N Ratio 15:1 = 120.96 x 15 = 1814.4 gram

We already have Carbon of 1260 gram = 1814.4 – 1260=554.4 gram Carbon needed

Now let’s calculate how much carbohydrate is required to neutralize the Nitrogen.

Sugar Contents = 42% Carbon
Molasses Contents = 22% Carbon

In 1kg Sugar, you will get 420 gram of Carbon & in 1kg of Molasses it will be 220 gram of Carbon

For maintaining C:N Ratio of 12:1, you need 191.52 gram of Carbon

You need Carbohydrate (Sugar) = approximately 460 grams
You need Carbohydrate (Molasses) = approximately 870 grams

C N Ratio Based on Total Ammonia Nitrogen (TAN)

Let’s consider the Total Ammonia Nitrogen (TAN) in the BIOFloc Tank is 2 PPM (Parts Per Million or Milligram Per Liter)

For a 10,000 Liter Tank it will be 10,000 x 2 = 20,000 mg TAN

Lets convert mg to gram = 20000 mg / 1000 = 20 g TAN

For C:N Ratio 10:1 = 10 x 20 = 200 gram Carbon needed
For C:N Ratio 12:1 = 12 x 20 = 240 gram Carbon needed
For C:N Ratio 15:1 = 15 x 20 = 300 gram Carbon needed (add 615gm sugar or 1230gm molasses)

How to Control Ammonia when C:N Ratio is 15:1

8 ppm Ammonia 10,000L Water = Add 2 Kg 460gm Sugar & Double for Molasses
4 ppm Ammonia 10,000L Water = Add 1 Kg 230gm Sugar & Double for Molasses
2 ppm Ammonia 10,000L Water = Add 615gm Sugar & Double for Molasses
1 ppm Ammonia 10,000L Water = Add 300gm Sugar & Double for Molasses
0.5 ppm Ammonia 10,000L Water = Add 200gm Sugar & Double for Molasses
0.03gm Sugar or 0.06gm Molasses required to manage 1ppm of Ammonia in 1 Liter Water
0.45gm Sugar or 0.90gm Molasses required to manage 1ppm of Ammonia in 15 Liter Water
1.74gm Sugar or 3.48gm Molasses required to manage 1ppm of Ammonia in 58 Liter Water

Final Words

BIOFloc system can be successful if you understand how to control Ammonia using an appropriate Carbon Nitrogen Ratio (CN Ratio). Its technology-based farming, failure of maintaining single criteria can create havoc in the BIOFloc System. Before you start your own BIOFloc Fish Farming, understand what is BIOFloc and how it works.