Fish Stocking Limit - for FO and FOWLR

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Well-known member
May 22, 2006


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

bio-load - The biological impact put (in this case) on the marine system. This comes from 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.


This is a hotly debated topic. There are many opinions. Keep in mind they are just that: opinions. When the reader comes across an opinion and, before it is applied, the reader should ask the person how they arrived at this opinion. My opinion comes from science, knowledge of the fish BOTH in the wild and in captivity AND having kept several (not just one) of the particular fish over the last 40 years.

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 is 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 us 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 from it, 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 the title of this section, the bio-load the fish (and its 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 does not€™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 decompose in the aquarium. It is usually a chuck of flesh or a whole organism totally consumed. The Butterflyfish food is spread around the aquarium and some (hopefully little) contributes to waste organics and thus adds to the bio-load. I hope the reader can clearly see that the two fishes and their maintenance, though the same size, have a different bio-load impact on the marine system or setup.

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 length of time which will not allow the ammonia and nitrite wastes to adversely affect the fish, there must be enough bacteria there to perform the function fast enough.


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. However, what is known is that improved filtration increases the system's bio-load capacity.

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 surfaces used as Aquascaping, put into the display. The top layer of substrate also holds these bacteria. The aquarium walls, plumbing, and other surfaces like refugium and sumps walls 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.

Another way to increase bio-load capacity is the removal of wastes before bacteria have to process/eat it. The protein skimmer is the single most advantageous piece of filtration performing such service. It removes wastes before the bacteria have a chance to break them down. Thus, instead of needing more bacteria to process wastes, the wastes are partly removed from the system.

Other removal methods include resins, absorbents (activated carbon), and other chemical filters. They are a means to remove wastes/nutrients and reduce the need for the bacteria present to process them. This increases the bio-load capacity of the marine system IF the aquarist is diligent at maintenance of such equipment and chemicals.

So filtration has two basic forms: increased biological capacity and exporting/removing products from the water.

Circulation - Now that there are enough bacteria, they have to get the wastes. Since bacteria do not€™t go to the wastes, the wastes have got to go to the bacteria. This is where circulation is important. 25 years ago this was not€™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. (NOTE: Circulation provides other benefits, this is just one).

Since these waste processing bacteria (nitrification) require oxygen, they compete for oxygen with the fish, cleanup crew and other nitrogen waste producers. The bacteria and marine life could (in the extreme case) reach a kind of equilibrium which balances oxygen needed by the fish and the oxygen needed by the bacteria to process fish wastes. Or on the other side of the equation; there is not enough available oxygen for the bacteria to multiple and quickly handle the wastes where the fish and other life forms are taking up the oxygen. 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. So, gas exchange is still important for this process to be efficient.

Speed - I list this strange word separately to emphasize that bacteria numbers isn't the only desire, 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 combined with the available circulation 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. If the bacteria are not given this chance to expand in numbers, wastes may go unprocessed, enter the water as a poison (poor water quality), and stress sensitive fishes. 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 will not fit into that marine system.

Some aspects of this include considerations given here: What Size Tank for that Marine Fish?. The bio-load capacity may be fine, but will the fish BOTH physically and mentally fit into the display tank?

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 do not 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 has not 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 cannot support such a fish.

Some fish also do not get along with other fishes. The bio-load capacity might be there, but putting a large predator in with a group of small fishes will usually not work. The hobbyist and aquarist may THINK they know when a fish is not getting along with another, but the reader would be surprised just how much 'aggression' can be subtle. It is not what is seen as much as it is what the fish perceives. A fish is MUCH more sensitive to a bully and does not need to be chased about to be kept in hiding. That shy fish might in fact be a fish that senses it is insecure with its tankmates.

To complicate things a bit further, there are fishes that have a combination of the above needs. Stocking limit is subject to the characteristics (physical and mental) 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 am not talking about religion for fish!:D) 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 unit length of fish. If you don't understand anything from this post, you should try to understand that: There is no direct (linear) correlation between fish length, and the bio-load produced. If you double the length of the fish, you DO NOT double the bio-load. If you double the length of the fish you DO NOT double the mass/weight. That is what not being linear means. Take this as an example: A 2 inch Lionfish DOES NOT make half the bio-load of a 4 inch Lionfish. The 4 inch Lionfish could actually produce more like a factor of 3 or 4 times the bio-load of the smaller fish. A 2 inch Lionfish DOES NOT have half the mass/weight of a 4 inch Lionfish, either. :)

Put the above knowledge together with the previous example of the Copperband and Lionfish, and you can see two examples of where length is not related or connected to a fish's bio-load on the system. The point here is that length has actually little to do with bio-load. The most important feature is fish mass/weight, when it comes to a fish stocking limit. BUT hobbyists know or can estimate the length not the mass of their fish. Length can be used, but the mass component has to be taken into account by describing the fish as 'lean' (Copperband) or 'thick' (Lionfish, Puffers, some Triggers) or somewhere in between (Tangs, Angels, some other Butterflyfishes, etc.).

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 its length. 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. (Are€™t you glad you read all this for that answer?) Take two opposing marine systems: The nano of 20 gallons and the 125 gallon (standard) display with 20 gallon sump, 10 gallon refugium, properly sized skimmer, abundance of live rock, and mechanical/chemical filter.

The Nano: With its low filtration capacity, it has a very low ability to biological support fish and cleanup crew wastes. It most likely does not have a skimmer, but may have some mechanical filtration that includes activated carbon. Next, the swimming space is not there. Next, the substrate space is not 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 aspect/consideration you might come to the conclusion that a 5 gallon nano should not contain any fish after putting in a cleanup crew. You would be right!

The 125 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 swimming length. Similarly, there is a goodly amount of substrate space and territory areas to accommodate most marine fishes. To recommend a stocking limit by length, a couple of conditions have to be put on the recommendation: Total water volume; and mass of the fish.

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 limit is reduced as the fish mass increases. For instance, if fishes like Tangs, some Triggers, Angels, and thicker Butterflyfishes (Raccoon) will be kept, a recommended stocking limit be more like 1 inch of fish per 4 gallons.

More mass per inch, more 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.

WARNING: Just because there is an identified stocking limit does not 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 (see: The Mature Aquarium) 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.

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 should their fish stock limit be, 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 about bio-load and bio-load capacity and fish behavior to provide a reasonable fish stocking limit.

If I don't have or get the information (but know it isn't a nano) and still make a recommendation, then I assume a 'medium' to 'low' filtration system, with medium mass fishes and use 1 inch of marine fish per 5 gallons.
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Well-known member
May 1, 2008
Coeur d'Alene, Idaho

Great information. Thank You! :)

"In the time that it takes a single Nitrosomonas cell to double in population, a single E. Coli bacterium would have produced a population exceeding 35 trillion cells."


"They must oxidize ammonia and nitrites for their energy needs and fix inorganic carbon dioxide (CO2) to fulfill their carbon requirements. They are largely non-motile and must colonize a surface (gravel, sand, synthetic biomedia, etc.) for optimum growth. They secrete a sticky slime matrix which they use to attach themselves."

I know I can install/build a filter to eliminate waste for any bio-load. It is the oxygen demand of the Nitrifying bacteria I do not think I can maintain, as the bacteria use more oxygen than the total tank load.

"Maximum nitrification rates will exist if dissolved oxygen (DO) levels exceed 80% saturation. Nitrification will not occur if DO concentrations drop to 2.0 mg/l (ppm) or less. Nitrobacter is more strongly affected by low DO than NITROSOMONAS."


"All species of nitrifying bacteria require a number of micronutrients. Most important among these is the need for phosphorus for ATP (Adenosine Tri-Phosphate) production. The conversion of ATP provides energy for cellular functions. Phosphorus is normally available to cells in the form of phosphates (PO4). Nitrobacter, especially, is unable to oxidize nitrite to nitrate in the absence of phosphates."


"The increasing popularity of high-tech water filters for deionizing, distilling, and reverse osmosis (hyper-filtration) produce water that is stripped of these nutrients. While these filters are generally excellent for producing high purity water, this water will also be inhibitory to nitrifying bacteria."

Full Article:

Also, I think the teritorialism of different species must also be a main concern when stocking.


"If this don't work, I can always change the water!"



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