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How Many Fish? – 4 Questions You Should Ask

Really how many fish? Replicating the stocking density of what others tell you they stock their fish at, is not as easy as it seems.  If you are provided with all the information about how the specified fish numbers were achieved, great.  If not, there is no doubt you will find it quite difficult to replicate.

How Many Fish can you Handle

I have given some basic guides in The Magical 100 Fish in 1000 Liters, written magazine articles on the topic and calculate the feed load in every commercial recirculating aquaculture systems we design.  As you expect the same question pops up for the new comer to backyard fish farming and aquaponics, how many fish can I stock.

The answer to how many fish can I have is made more complicated for the beginner, with poorly written articles showing up on the net that suggests a recommended stocking density of “40 fish in a 55 gallon drum”.

While we commend the authors for their efforts in growing food at home, the article itself is lacking critical information for the reader to successfully replicate the outcome.

[blockquote cite=”Tom Losordo” type=”left”]Don’t be impressed with high fish densities. Fish can be held at high density with low feed, even in a poorly designed system[/blockquote]

Questions you should ask

Answers to these questions should be very specific and to the point.   Vague ramblings are less than useful and often results of following such advise is quite disastrous.  If you can not get exact answers from authors of the stocking density recommendation, it is in your best interest to move on to an alternate source of info.  There are no pieces of string when it comes to how many fish you can keep.

1 – How much fish feed per day and what feed are you using?

This is the most important question.  Because all the oxygen consumption and nitrogen production come from the fish feed. (two of the most common limiting factors in fish farms). These are the first two things that will kill your fish, lack of oxygen and or too much nitrogen in the water.

Answer you might look for:
On average X grams of X% protein feed per day or
X grams of other feed sources per day

2 – What is your pumping rate through the fish tank (fish tank exchange rate)?

Pumping through a fish tank will carry the fish poop, wasted feed and nitrogen (Ammonia and Ammonium) wastes out of the fish tank before they build up to toxic levels.  We are hoping that water is being put through some form of filtration (we will cover filters in a minute).

Answer that is ideal:
I am exchanging the fish tank water X times per hour

3 – Do you have aeration if so how much and are you using air stones, what size?

We are managing the primary limiting factors we mentioned in the first question (Oxygen and Nitrogen).  The pumping is dealing with the Nitrogen and aeration will be sorting out the Oxygen.  Keep in mind aeration can be provided by pumping water (spray bars and venturis for example).

Answer that will be handy:
I have X litres per minute air pump with X litres per minute air stones in each tank.

4 – Have you got biological filtration, how much and what type?

We move the nitrogen wastes out of the fish tank but we need to filter it before we send it back to remove the toxic nitrogen.  So we will need some biological filtration to convert the Total Ammonia Nitrogen (TAN) from the fish into Nitrate using bacteria.  If there is no filtration, you would have to ask quite a few other questions but let’s stick to the “norm”.

Answers you are looking for:
I have X amount of X size X type of media (could be gravel or specific bio media).  If specific surface area info is available, get that too.

4 Simple Questions will Save Your Fish

So next time you are out there cruising the internet  looking for how many fish you can keep in your drum, barrel or fish tank, these 4 questions may save you some heartache and loss of fish.

  1. How much feed per day and what feed are you using?
  2. What is your pumping rate through the fish tank (fish tank exchange rate)?
  3. Do you have aeration if so how much and are you using air stones, what size?
  4. Have you got biological filtration, how much and what type?

The authors that are offering stocking density advice or  saying you can keep X amount of fish per liter or gallon will be more than happy to answer a few simple questions accurately to help you make sure you can get the same, great results they do.

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