Marine Fish Stocking Limit

Discussion in 'Fish Diseases & Treatments' started by leebca, Nov 20, 2007.

  1. leebca

    leebca New Member

    Jul 4, 2007
    Likes Received:
    So CA

    Prologue: I think I have a way most readers can have a handle on marine fish stocking limit. It is not definitive, but can give some guidance.


    Best to begin this post with some common ground. Maybe not perfect, but let’s agree on some basics for this topic:

    bio-load - The biological impact put (in this case) onto the marine system. This comes from nitrogen waste sources, including but not limited to: fish wastes (gas, solids, liquids), wastes gases and nutrients by bacteria in the filter, detritus, and feeding on organics, microbes, macro plants in the system, snails (cleanup crew), mobile invertebrates, etc. and the nutrient takers, including but not limited to: fish (taking up oxygen); bacteria (taking up oxygen); plants; microbes; etc.

    bio-load capacity - The limit of bio-load that a particular marine system can handle fast enough to keep poisons from affecting the health and well being of the marine life.

    fish length - Measuring from the tip of the mouth (‘nose’) to the base of the tail, not including the tail. The base of the tail area is known as the caudal peduncle.

    nitrogen wastes - Throughout this post you'll find the word 'waste.' What is really most important is the nitrogen waste in relation to how the system can cope. Other wastes are usually dealt with by other organisms and hopefully the skimmer, water changes, mechanical and chemical filtrations.


    This is a hotly debated topic. There are many opinions. The tendency is to express a stocking limit in terms of fish length. After all, this is how it was done in freshwater systems; it is easy for the new hobbyist; and, depending upon who’s doing the recommendation, has a large range (e.g., someone selling fish to make money may suggest the buyer put into the system many fish, above common stocking limits). Is there any kind of ‘easy’ system that works? Let’s see what has to be considered.


    Putting together a stocking limit recommendation has to take several factors into consideration, not all of which are easily measurable and some of which have within themselves some opinions and variations. Nonetheless the things that do need to be considered can be split into three general groups:

    Bio-load - The wastes produced and nutrients taken up by the living organisms in the closed marine system contribute directly to the fish stocking limit.

    Bio-load capacity - The capacity of the marine system to handle wastes put into it and nutrients taken away is a prime factor to the fish stocking limit.

    Fish behavior - By this I mean what the fish needs in the way of swimming space, substrate space, or whatever life it has evolved to in the ocean. This section also includes compatibility.


    This is not easily measured. But in truth, this is not just the length of the fish. It is more in tune with how much waste the fish contributes to the system. Or, in keeping with this section’s title, the bio-load the fish (and it’s maintenance) puts on the system.

    For instance, take two fishes: A growing Copperband Butterflyfish, and a full sized but growing Lionfish. Now, let each be the same length = 4 inches. Will they each have the same bio-load on the system? No. The Lionfish, being a predatory carnivore will make large waste deposits but not frequently to the system. In between feedings, it doesn’t swim much and requires little swimming space. The system must be able to handle such large ‘shots’ of waste. The Butterflyfish is lean and eats throughout the day producing little wastes, but frequently. The Butterflyfish bio-load is lower, requiring the biological filter to have a minimum supply of bacteria.

    The Lionfish food is not left to rot in the aquarium. It is usually a chunk of flesh or a whole organism totally consumed. The Butterflyfish food is spread around the aquarium and some of it contributes to waste organics and thus adds to the bio-load. I hope the reader can clearly see that the two fishes, though the same length, have a different bio-load.

    With the addition of each fish to the system comes the corresponding increase in bacteria and microbe numbers to handle the familiar wastes of ammonia and nitrites, but also the number and kinds of bacteria and microbes that eat/process/handle organic wastes. The type of fish brings with it a different impact on the numbers of these bacteria that are needed. Taking the example, the Lionfish will have a large impact on these other microbes when it ‘dumps’ its package of waste. The Butterflyfish will not have such a large impact.

    In order for the wastes from these fishes to be processed by the bacteria in a time which will not allow the ammonia and nitrite wastes to adversely affect the fish, there must be enough bacteria there to perform the fast function.


    The bio-load placed on the marine system must be processed in the marine system fast enough to not poison the fish. There is a limit to how much wastes can be processed and the nutrients the bacteria have access to, in order to perform their function. Thus begins the concept of a bio-load capacity of the marine system. How can this be increased and how can it be assessed?

    Filtration - This is one of the foundations of the bio-load capacity of the marine system. 25 years ago we had poor filtration systems that kept the stock limit very low. An example was that most would recommend about 1 inch of fish for every 10 gallons of system water. As filtration improved, the number was moved to 1 inch of fish every 6 gallons. Today, no one has done work to update this relationship to current filtration types and the combination of these types.

    What is important in biological filtration (nitrification) is to provide a lot of surface area for the bacteria to quickly process the wastes. The closer the bacteria are to the source of the waste (closer to the fish, cleanup crew, etc.) the faster they can handle it. This is one advantage of live rock or any rough surface put into the aquarium. The top layer of substrate holds these bacteria. The aquarium walls, plumbing, and other surfaces like refugium and sumps hold more bacteria. Thus adding such features to the marine system increases the numbers of such bacteria and thus increases bio-load capacity.

    Mechanical filters may also be added to the system to help process wastes. Not just collection points, they may also become a part of the biological filter. When they are so used, their ‘cleaning’ has to be done carefully to maintain their biological activity. BUT don't misunderstand me. A true mechanical filter should not become a biological filter. This filter, not maintained properly, turns into a biological filter.

    Another approach is the addition to the system a means to remove the nutrients that the bacteria feed upon. The protein skimmer is the single most advantageous piece of ‘filtration’ performing such service. It removes organic wastes before the bacteria have a chance to break them down (eat it). Thus, instead of needing more bacteria to process these wastes, the wastes are partially removed.

    Resins, absorbents (activated carbon), and other chemical filters work to remove wastes, like the skimmer, before it becomes food for microbes that contribute to the bio-load. Using a carbon filter with the skimmer is necessary for most new to the hobby. Having these working properly increases the bio-load capacity of the marine system IF the aquarist is diligent at maintenance of such equipment and chemicals.

    So 'filtration' in a generic sort of way, has two basic forms: increased biological capacity and removing wastes.

    Circulation - Now that there are enough bacteria, they have to get the wastes. Since they don’t go to the wastes that means the wastes have got to go to them. This is where circulation is important. 25 years ago this wasn’t even a part of the system beyond what was needed to keep the water surface in motion to help exchange gases (at least we knew about gas exchange back then). If live rock and substrate are where the bacteria reside to process wastes, then the water must move sufficiently to bring the wastes to those areas of the system. Usually this is in the display tank and refugium. So water flow rate in these areas is of major concern. A faster flow rate will increase the bio-load capacity of the system, up to the point where anything much higher has little impact. That number is about 10 turnovers per hour for the total water volume in a FOWLR system.

    Since these bacteria require oxygen, they compete for oxygen with the fish, cleanup crew and other nitrogen waste producers. The bacteria and marine life reach a kind of equilibrium which says that more fish can’t breath well because the bacteria their wastes require need more oxygen. Or on the other side of the equation; there isn’t enough available oxygen for the bacteria to multiple and quickly handle the wastes. This is uncommon, but still a factor in the bio-load capacity. This is also the importance of circulation. Not only are the wastes needed to get to the bacteria, but the other nutrients (e.g. oxygen) needs to get to the bacteria too.

    Speed - I list this strange word separately to point out that it just isn’t the numbers of bacteria that process wastes which makes for a high bio-load capacity; but it is also how fast the bacteria can process the wastes. Wastes that poison and pollute the water must be separated from sensitive fishes quickly or the fish may be harmed. The speed at which these bacteria process the wastes is important to the over-all bio-load capacity, as well as their right number. Usually to handle the speed portion, the number of bacteria have to increase beyond a long term need and instead must reach an inflated number sufficient enough to process wastes quickly. This is a prime reason why fish should be added to an established system slowly, not all at once or quickly one after another. The established system requires time for the bacteria to ‘catch up’ to the increased bio-load and may endanger current inhabitants if they can’t respond to the bio-load or kill the new additions. What goes wrong when waste producers are quickly added to the system, is poisons aren't processed fast enough, leading to poor water quality. The result is that the old and new fish are stressed – sometimes to death.


    Now we get to the area that, despite the bio-load measurements, the fish just won’t ‘fit’ into that marine system.

    Some fish need to stake out territory. Even if the system can handle the bio-load, is there enough territory for the fish to call its own?

    Some fish need swimming space. The Tang was given as an example early on. They need tank length. So even though a small marine system may handle the bio-load from a Tang, it may not have enough swimming length/space. Most hobbyists don’t understand that large marine Angelfishes need swimming space, too. These fishes look attractive, but they belong in large tanks with plenty of swimming space which most hobbyists only dream about.

    Some fish need substrate space. Putting a sand sifter that hasn’t been trained to eat prepared foods into an aquarium with little or no or the incorrect type of substrate is dooming the fish to death, even though the bio-load capacity of the system can handle the fish wastes. Such a system can’t support such a fish without supplementing the fish's diet with prepared foods.

    Some fish also don’t get along with other fishes. The bio-load capacity might be there, but putting a large Puffer in with a cleanup crew of shrimps will usually not work.

    To complicate things a bit further, there are fishes that have a combination of the above needs. Stocking limit is subject to the characteristics and/or behavior of the fish one wants to keep.

    The reader may come to some conclusion from this post on how many inches of certain kinds of fish fit into a number of gallons of certain marine system’s total water, BUT the above factors must still be taken into account.


    A few odds and ends need to be pointed out with regards to the fish stocking limit.

    Fish mass - (I’m not talking about religion for fish!) Each fish has a mass. You might want to think of this as weight. The heavier the fish, the more wastes it is likely to produce per inch of fish. There is not a 'linear' relationship between weight, length, and bio-load. That is to say that a fat 4 inch fish doesn't have twice the bio-load of a fat 2 inch fish. In fact, the 4 inch fish might have 3 or 4 times the bio-load of the 2 inch fish!.

    Going back to the example of comparing the Copperband Butterflyfish to a Lionfish, the fish may be the same length, but they have distinctly different masses. It turns out that mass/weight is more important than just the length of the fish when recommending fish stocking limits.

    Eating habits - Already mentioned by example, the general statement is that predatory carnivores produce large packages of wastes. Other carnivores are also large waste producers. Herbivores also can produce a lot of wastes, but of a different sort. By all this I mean just how ‘messy’ a fish is in captive life.

    The bigger picture - Put it all together and what to you get: Bio-load impact by a fish depends on the kind of fish it is (eating habits, mass/weight, quantity of waste produced, etc.) not only on how long it is. The bio-load capacity of a marine system depends on the configuration (mostly how much and the kind(s) of filtration and circulation is provided), equipment and upkeep the system has, as well as the size and make-up of the cleanup crew. This now should be clear on why there are so many different opinions about fish stocking limits. There are a lot of variables.


    Is there a simple, easy means to recommend fish stocking limits? No. (Aren’t you glad you read all this for that answer?) Take two opposing marine systems: 1) The 'basic' nano of 20 gallons, and 2) the 125 gallon (standard) display with 20 gallon sump, 10 gallon refugium, properly sized skimmer, abundance of live rock, and mechanical/chemical filter.

    1) The Nano: With its low filtration capacity, it has a very low ability to biological support fish and cleanup crew wastes. It most likely doesn’t have a skimmer, but may have some mechanical filtration that includes activated carbon. Next, the swimming space isn’t there. Next, the substrate space isn’t there. So although fish can be put into this system, they have to be very limited in both number and type of fish. Since these fishes are generally lean, short fish, a length standard may be applied. I would apply 1 inch of fish per 5 gallons of actual water volume IF live rock is in the tank. The fish length is based upon the ‘normal’ full size the fish reaches, NOT its current size. If you look into this ‘direction’ you might come to the conclusion that a 5 gallon nano with cleanup crew should not contain any fish. You’d be right! So, this 20 nano has about 16 gallons of water max (tank isn't full and it has rock, decorations, and substrate). It can handle a max of 3" of fish. BUT that is a full grown, 3" thin fish, not a fat fish or a fish with a large mass. This means one very small Goby that doesn't grow any larger than 3" when full grown. Not a very satisfying fish tank. This would make a great reef tank (no fish)!

    2) The 125g display: This system has a very large bio-load capacity by virtue of its filtration and add-on water treatments. Further, the length of this display can accommodate some types of fishes (like Tangs, but NOT large Angels) that need swim length. Similarly, there is a goodly amount of substrate space and territory areas to accommodate most marine fishes. Assuming there is about 135 gallons of water in the system AND assuming lean fishes, the fish stocking limit might be 1 inch of fish per 3 gallons. This has to be factored down as the fish mass increases. For instance, if fish like Tangs, Triggers, bulky Wrasses, etc. will be kept, it might be more like 1 inch of fish per 4 gallons. If Puffers (probably no more than two), full sized Lionfish or other predatory fishes will be kept, that is more like 1 inch of fish per 6 gallons. The estimated maximum length of the fish needs to be used, not just its current length. Fish need bio-load capacity to grow, as well as space. The above numbers assume a clean up crew of snails and worms.

    What needs to be stated clearly about keeping marine fish is this: Any tank below 10 gallons is not acceptable; a 10 to 15 gallon tank can hold one fish -- a tiny Goby (and will NOT hold any Anemonefish (Clownfish)). If you have fallen in love with the idea of having an Anemonefish (Clownfish), then see the next section.

    This is their sizing for tanks: If you have one of the small species (that grows to be no more than 3.5 inches in length at full adulthood), then a 20 gallon tank is marginally good. These fish are 'fat' (not thin) and have a medium body mass. If you want to have a pair of these smaller species, plan on a 29 gallon tank or larger. Both of these assume there are no other fish with them. The 'pair' tank size isn't just from their bio-load, it is also from knowing that in the ocean, a pair will stake out a territory of about about 7 cubic feet of water. NOW! If you are interested in the larger Anemonefishes, like the Maroon (the female will grow to over 6"), the tank size for one is 29. A pair will need a minimum of a 45 gallon tank, but what I suggest is the standard 60. In this larger tank, you could still add a third or fourth fish that doesn't pose a threat to the pair. The pair would like to claim 9 cubic feet of tank space as their own.

    The above numbers assume an average system (skimmer, carbon, mechanical filter, proper circulation, live rock or sufficient biological filtration, lighting and very good water quality) with a clean up crew which also adds to the bioload.

    What gets particularly complex in determining the fish stock limit is: when the clean up crew has diverse members (other than snails and worms); when the fish are mixed -- some Butterflyfishes with Tangs and Angels, for example; and when the marine system has some but not all of the optimizing equipment. :eek:

    WARNING: Just because there is an identified stocking limit doesn’t mean the hobbyist has to stock up to that limit. If the hobbyist is just starting out, especially when the system hasn't totally matured then keep well below the stock limit. As time and experience increases, the stock limit may be slowly approached. This will lessen the unnecessary loss of marine life. :thumbup:

    These two examples should put the reader in the ball park of piecing together a reasonable fish stock limit to their system. The next time someone asks what their fish stock limit is, ask:
    What kind of fish?
    What fish are there now?
    What other waste producers are in the system?
    What kind of marine system?
    Maintenance routine?
    and ask for all the details outlined above as to what is important to the best reasonable answer.

    If I don't have or get the information (but know it isn't a nano) and still make a fish load recommendation, then I assume a 'medium' to 'low' filtration system, with medium to small mass fishes.
    Last edited: May 18, 2011
    6 people like this.
  2. BigAl07

    BigAl07 Administrator

    Mar 1, 2007
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    Waynesville, North Carolina USA
    Excellent write up!!!
  3. Woodstock

    Woodstock The Wand Geek was here. ;)

    Nov 20, 2003
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    North Carolina
    You got Karma :D Thanks Lee!
  4. nightfire76

    nightfire76 New Member

    Aug 13, 2009
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    that was very well put
  5. Rhodes19

    Rhodes19 New Member

    Jul 1, 2009
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    Charleston, SC
    Very well written and easy to read and understand. :) Thanks for doing that.
  6. DrHank

    DrHank New Member

    Apr 30, 2007
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    Crestview, FL
    Terrific! If folks would read and understand what you are saying here, a lot of unnecessary loss of fishes could be avoided. An excellent dissertation.
  7. DianaKay

    DianaKay Princess Diana

    Jan 4, 2013
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    Oklahoma, USA
    Reef Chronicle:
    As we've had ??'s ask on this topic....thought I'd :bump: bump this older thread UP. :)
  8. Snid

    Snid New Member

    Apr 11, 2014
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    Ohio USA
    Reef Chronicle:
    Nice. This clearly explains why all my research on how many fish I can plan for in my new tank has varying answers. I am going to safely plan on a little less than what I calculate as a safe quantity and then pay close attention to how the tank handles the bio-load. Of course, this process will be one or two fish at a time per month with a quarantine tank and a reasonable grouping and order of introduction.
  9. Supermookik

    Supermookik New Member

    Dec 15, 2014
    Likes Received:
    Los Angeles
    This was an awesome write up thank you!!
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