Basics : Stocking Guidelines
Why one inch of fish per gallon? What are the purposes of stocking guidelines? On this page I offer a variety of points to consider on this highly subjective topic. There is no single inclusive magic formula, and after reading this article, you will have no need for one. There are many ways to plan or assess stocking levels.
Like anything else, the more familiar you are with the factors, the easier and faster it is to estimate safe stocking levels. Experienced hobbyists do this at a glance without really thinking about it, and the rest of us do a little research and planning. In the final analysis, you might find that it's much more pleasant to have less species but more of each type, in a larger aquarium, which ends up being less work for you, than a smaller more crowded aquarium.
Details...
The scientific methods
below (industry standard by volume, surface area, biological capacity
and mechanical filtration) all strive to use straight mathematical formulas,
with a correction factor for size. The correction factor is needed because
a fish's biological 'footprint' increases much faster than its length.
For example a 20" fish will eat many times more than 20 1" fish. Even
a 4" fish can be expected to eat much more than two 2" fish (and what
they eat is a measure of what needs to be filtered out of the water),
so some method is needed to correlate size and length into the formulas.
One method for bio-mass comparison is to cube their length (and as applicable,
apply a multiplier to compensate for different body shapes). This would
make 125 2" Cardinal tetras (125 x 2 x 2 x 2 = 1000) to be the biological
equivalent of one 10" Oscar (1 x 10 x 10 x 10 =1000). Scientifically,
mass (or biological footprint) is one of the most important aspects
in calculations, correlating both food intake and oxygen usage.
Industry Standards
This industry standard 'fish-load' calculation is based on inches of length and US gallons of water. For Goldfish, Oscars and large Plecos, allow 3 gallons of water per inch of fish (not including their tail). For all other small to medium sized tropical fish, allow 1 gallon of water per inch of fish (again does not include tail length in calculation). This standard is loosely associated with manufacturer's filter ratings, which also have a similar alignment to water volume. Although filter ratings would be more sensible if they were aligned to food or fish weight, this is impractical, so when a filter is said to be rated for a 20 gallon tank, this is really about biological capacity, and assumes the industry fish-load calculation above, is not exceeded.
Both Axelrod & Dr Shultz published "..in an aquarium which is wider than it is tall, allow 1 inch (of body) of fish per gallon of water. Under constant aeration, this can be tripled", so under more optimal conditions, these authors felt 3" per gallon was feasible.
The more you learn about fish, the less this method makes sense, but it's good general starting point, intended to keep beginners out of trouble. Also note that it quickly looses applicability as the tank sizes get much larger (70g+), but by then, hopefully the hobbyist has a better grasp of fish loading.
Surface Calculations
Surface calculations have everything to do with the rate at which a body of water will re-oxygenate. It starts by assuming the tank's depth is not greater than its width (which is often untrue now), as deep tanks behave very differently from shallow tanks. Many formulas exist, with a correction factor for the size of the fish. For correction factor, Innes used a logarithmic graph to show the relationship between surface area needed and size of fish to correlate quantity. For beginners, he used 3 sq.in. for Guppies, 18 for Swordtails and Platys and 20 for Barbs and of a similar 3-1/2" size. Another correlation I read was that for any fish under 2", allow 6 sq.in per inch of fish. For 2 to 4" fish, allow 9 sq.in. per inch of fish. For 5 to 10" fish, allow 12 sq.in per inch of fish. Another proposal by Dr. Cliff W. Emmens used the following table.
| Fish Length | Fish/1000 | cm2 per |
| (cm) | cm2 surface | fish |
| 1 | 400 | 2.5 |
| 2 | 110 | 9 |
| 3 | 50 | 20 |
| 4 | 25 | 40 |
| 5 | 12 | 80 |
| 6 | 8 | 125 |
| 8 | 5 | 200 |
| 10 | 2 | 500 |
| 12 | 1 | 750 |
| 15 | 1 | 1100 |
From 'The Advanced Aquarist', I think Dr.Ghadially summed up the surface method nicely: "There is no way of accurately calculating the size of the surface area needed per inch or per gram of fish. There are just too many variables to permit this to be done even reasonably accurately. Further, a surface area adequate for a given species or size of fish at the moment may be inadequate for another species or prevent growth to its full size and capacity."
Several variables interfere with surface calculations. Water temperature has a direct effect on water's ability to hold oxygen (colder water holds more oxygen). Oxygen diffusion is another factor. With no filters and no aeration, only the surface is well oxygenated. With modern filtration providing a constant turbulence crossing the surface and dropping down into the tank, then re-oxygenation is rarely an issue in anything but the most severely overstocked tanks. However, during a power failure, the fish will exhaust the supply of available oxygen and then become dependant on the surface rate of re-oxygenation. If you are in an area with frequent and or prolonged power failures, then you would stock lightly, or have backup facilities.
Another consideration around re-oxygenation is that not all fish have
the same dependence on oxygen. Large active Characidae such as Silver
Dollars would suffer low oxygen levels much sooner than Anabantidae
such as Paradisefish & Gouramis, with their labyrinth air breathing
organs. Some catfish also have auxiliary air breathing capabilities,
while other catfish do not, and suffer very quickly as the oxygen levels
deep in the water are never as good as the surface.
Biological Capacity
This is probably one of the easiest to understand, and hardest to quantify, until symptoms begin appearing. Approaching the limits can be hazardous for your fish's health. Basically as fish have no toilet to take away their wastes, an aquarium's balance pivots around it's capability to have these wastes converted into nontoxic forms, as quickly as they are produced. Plants do a significant amount of the work in any aquarium which has a high plant to fish ratio, but more typically, the task falls to our filters which provide the colonizing space for nitrifying bacteria.
For example, the AquaClear 200 and Penguin 170 filters are both optimistically
rated for a 50 gallon tank. Manufacturers rate their filters by the
amount of solid waste the filter will hold, the amount of flow they
generate (to adequately circulate the water) and most importantly, how
much bacterial capacity does the filter have. This bacterial capacity
is then correlated to the industry standard fish-load which will be
found in a 50g tank. The AC200 achieves this bacterial level with a
relatively huge surface area in its sponge, while the P170 uses a biowheel
to augment its much smaller sponge to achieve the same capacity rating.
The bottom line is that a filter should really be sized according to
the amount of food being added to the aquarium, as the food starts the
chain of events (more food = more waste = more bacteria = bigger filter).
After a tank has 'cycled' (over 8 weeks old and is in stable operation), then any ammonia or nitrite measurements would suggest the filters were improperly cleaned (bacterial capability damaged) or the aquarium is under-filtered, or more specifically, the filters have insufficient bacterial capacity for the current bio-load. This can be easily corrected by feeding less (which decreases the amount of waste being produced), or by adding more filtration (another filter, or another sponge into the AquaClear).
While an aquarium can have too much turbulence, it cannot be overfiltered,
however overfiltering to compensate for too heavy a bio-load is problematic.
Whenever too much reliance falls on any single component, your margin
of safety decreases and your risk of failure increases. In the case
of an overworked filter, the smaller operating margin would cause mini-spikes
of ammonia whenever the filter was serviced. If a power failure went
on long enough to result in the death of the bacteria, this would leave
an overcrowded tank to quickly fall prey to the build up of toxins.
Generally, you should overfilter, by using more than one filter for operating redundancy. Each filter would be sized for between 65 and 100% of the tank's requirements, such that any failure could be carried for a short term by the remaining filter.
As fish waste is plant fertilizer, growing plants will always provide
some filtration by removing ammonia, nitrites and nitrates, and storing
them for energy later, or converting them to cellular growth. Whether
the plants can do this to any significant level, really depends on the
fish-plant ratio. As it takes a very high ratio, the typical aquarium
relies on bacteria to take the wastes from ammonia through to nitrates,
and then, to some degree, the plants feed of the remaining nitrates.
Mechanical Filtration
Not typically a constraining factor, but we rely on filters to remove solid waste, by having enough turbulence to pick up the various detritus which collects on the substrate. These solids are then either removed from the system by cleaning or backwashing the filter, or the are left in the filter where they undesirably decompose into dissolved organic compounds DOCs.
If you have a lot of breakwater (rocks, plants, ornaments) and very poor circulation, then all these solids (besides being unsightly) will add to the level of DOCs in the tank. Under this operating mode, the tank's limits would be quickly reached by a few large fish, and require very large frequent water changes to keep the DOCs and nitrates low, and to prevent build up of toxins such as ammonia and nitrites.
The physical and behavioural
guidelines below (diet, feeding zones, swimming area etc) are an acknowledgment
that there are some limits as to how much we can mix fish from completely
different environments with different characteristics. These are probably
the most valid set of guidelines which limit stocking (while at the
same time, not exceeding the others, with a few exceptions).
Diet
Although not considered a typical stocking limiter, diet can definitely
influence how many of what type of fish you will be able to keep together.
While most fish are somewhat omnivorous, many will not be able to digest
large amounts of a high protein menu, as their long intestines are designed
for the slow digestion associated with breaking down plant matter. Accordingly,
when mixing herbivores with carnivores, the menu must be brought down
to the herbivore level, which may constrain the growth of the carnivores
to some extent. This is especially prevalent when mixing African cichlids.
The name Malawi Bloat has been associated to a wasting disease which
herbivorous may acquire from eating inappropriate foods.
Feeding Zones
Another aspect to consider is feeding zones. Aggressive top-feeders
may starve the more passive fish into poor health and eventual death.
This is easily demonstrated with a tank of common Zebra danios and some
Corydoras catfish. The danio's feeding method is comparable to the feeding
frenzy of a pack of Piranhas, and getting a flake food past the danios
to the waiting catfish can be challenging (use bottom feeder tablets).
There are many instances when dissimilar eaters will restrict the types
or quantities of fish being mixed together.
Swimming Area
Area of activity, and metabolism governs where the tank will be filled from, and how busy it will be. As a general guideline, an overstocked tank with fish distributed to every level will fare much better than a tank with the same number of fish, all residing at the surface or the substrate. When stocking a mixed community tank, consider how many fish will be bottom dwellers, mid dwellers and top dwellers and try to select so that they are spread out.
An aquarium can be well stocked with fast moving high adrenaline fish, or well stocked with slow passive fish, but mixing the two in a dense population would be problematic. Continuously dodging or being dodged would be stressful for some, if not all the inhabitants. Either stock lightly, or keep to one side or the other of the activity scale.
Compatibility
An example of incompatible fish would be a couple of large cichlids,
such as a new world cichlid from Central or South America and an old
world cichlid from the African Rift lakes. At a glance, a Jack Dempsey
and a Frontosa would not appear to be incompatible, having similar diets
and being given a large enough tank while they were similar sizes. The
Frontosa fades his colours to show aggression, while the Jack Dempsey,
for the same emotion, increase his colours. They simply do not share
a common language when it comes to this method of communication. Each
would communicate aggression or submission in the opposite manner being
understood, so the result would be confused, probably leading to conflict.
Most incompatibilities are much more obvious, such as fin nippers (Tiger
barbs, some tetras and danios), temperamental fish (many Anabantidae
and Cichlidae have well deserved reputations), and established personalities
(with age, some fish develop deep intolerance to the presence of others).
Territorial Requirements
Almost all fish will exhibit some territorial or alpha dominant behaviour.
While this is most pronounced at spawning time, many fish exercise a
high degree of territorial dominance throughout their lives. Fortunately,
the worst cases have very specific targets. Sharks (black, redtail and
rainbow) can be extremely intolerant of each other, especially in small
tanks, with varying degrees of acceptance towards other fish. The Siamese
fighting fish (Betta) is a classic example of a fish which claims the
aquarium as his territory and is always ready to defend it against potential
competitors (all other male Betta, or any fish with distracting colouration
such as male Guppies). Even when not spawning, many fish from the families
of Anabantidae and Cichlidae adhere to a pecking order, which as a minimum
defines the alpha male of the tank, (and often the alpha female).
Some territorial fish will centre their activities around a piece of shelter to defend (ie: a cave), so providing this shelter off into a corner is an option often used. Another option is to not provide the cave at all, removing incentive by not having anything to fight over.
There isn't a good middle ground when stocking a tank with territorial
fish. The two extremes are what work best. Either have the tank so lightly
loaded that they feel that their territories are protected and not under
threat, or have so many fish together that no zone, shelter or alpha
dominance will be stable and easily protected. This overcrowding technique
is often used with highly aggressive and territorial mbuna cichlids
to maintain a very dynamic pecking order. Under these conditions, there
are so many fish crowding for the alpha position, that the alpha male
risks losing his dominance if he is weakened or damaged in a fight,
or the other fish gang up on him, so he may not be as intent at confrontation
leading to death as he would have been otherwise, in tank with less
competitors.
Aggression
The African mbuna are well known for their aggression, which originates from their eating only as well as the amount of algae covered rocks they can protect. Fish are not ordinarily aggressive without cause, either due to a perceived threat to their dominance, territory, food supply, swimming space, mating competitors etc. Large old cichlids can be very aggressive or passive towards other fish, sometimes for no apparent reasons. When stocking aggressive fish, try to keep them all at the same level. Melanachromis auratus can work in a large enough tank with equally aggressive or sturdy fish, but mixing in some passive fish will result in a slaughter at some point. Tiger barbs, most sharks, Leporinus, large gouramis and many cichlids will fall into the medium aggression category. Large new world cichlids, mbuna and Piranhas fall into the high aggression category, while most small fish are low aggression. Generally, size has a significant influence on potential aggression.
Species Overlap
Even when mixing compatible community type fish, each specie will tend
to occupy a portion of the tank. If the portions do not overlap (top-feeders
and bottom-feeders) then behaviourally, their lack of interaction and
non-shared living area will make them almost invisible to each other,
so they would generally ignore each other's presence. If you were to
have a group of Rummy Nose, Black Phantoms, Neon tetras and Harlequin
rasboras, then your tank would become crowded very quickly. These fish
all share the mid-water, and with different body shapes, they will tend
not to share their 'personal' area. If the mix was Pristellas, Bleeding
hearts, Serpaes and Lemon tetras, then their common shape would make
it much more likely that they would share their space, so they would
not get crowded as quickly.
Generally, try to stock with a limited number of species. Too many species overlapping and you will consume tank space too quickly. More fish of less species will also provide you with more representative and entertaining behaviour.
Species Limiting
Shoaling or schooling fish find comfort and safety in large groups,
so it is much more difficult to overstock an aquarium with one of these
species. Provided the filtration is adequate, fish loads of 10" per
gallon are achievable. An example would be 600 Neons tetras in a 60
gallon tank. Besides the other obvious tetras, danios and rasboras which
crowd well together, other fish who share this willingness to be in
a crowd are loaches (to a varying degree with Kuhlis being the most
amenable), Corydoras, Guppies, Platys and many small barbs. Moving into
much larger tanks, Monos, Silver dollars, Pacus, Piranhas and Tinfoil
barbs are all pack fish as well. An example of species limiting in a
50 gallon tank might be that your total of 18 fish over 3 species becomes
a total of 24 fish if there was only 2 species, or a total of 40, for
only 1 species.
While not immediately evident, the other factors below (spawning, growth rates and eventual size) will significantly influence and limit our early choices:
Spawning Urges
Everything about the average aquarium is wrong for spawning. Mate selection
is determined by who is in the tank, a spawning site is what's available,
privacy does not exist, and security is making the best of the situation.
Despite this the spawning urge is too strong for many fish to ignore.
It is somewhat remarkable that many fish can adapt to such non-ideal
circumstances which would never occur in nature. Space is probably the
most obvious problem, as any fish which exhibits parental care will
need to secure an area for the fry, and this zone will crowd the tank's
other inhabitants. Spawning behaviour will manifest itself in a variety
of ways, depending on species.
With livebearers, once sexually mature, the males will almost constantly
chase the females (so we try to have have more females than males, to
spread the stress). If there are no females, the males will sometimes
harass each other, and if the livebearer is alone, they can become a
nuisance to other fish, especially with larger Mollies and Swordtails.
There is some variation with egg scatterers. The males of the smaller
species may be persistent, but harmless, while larger species may stress
the females in the close confines of an aquarium. Here the correct ratio
is usually to have more males, and as shoaling fish, you want to keep
groups of 5 or more.
Nest builders go through a pre-mating procedure to assess compatibility.
If they are not compatible, then they might need to be separated for
safety. With these fish, there is usually only one parent doing any
childraising, so the other is usually removed. Other fish are chased
away from the vicinity of the nest and the fry, however, most will not
spawn in a community tank setting.
Mouthbrooders will spawn relatively quickly somewhere and go back to business. These are already mostly aggressive fish and the selection of mates, establishing the pecking order and holding the alpha male position can all be very hard on tankmates. There are exception, so always research your particular fish.
Leaf and substrate spawners are parental. The alpha male must be established, with male battles of varying intensity. In some cases, an alpha female has also to be established, sometimes with as much battling as the males. Mate selection proceeds and then a zone is secured. Rejected mates will sometimes risk death if left in the aquarium. The zone can be as little as a cubic foot around the nest, but it's usually much larger, stopping when the rest of the tank's inhabitants have no room left, or in an extreme case, by killing the tank's inhabitants.
It is not unusual to have a balanced community tank come undone by fights for pecking order, or for the breeding needs of some cichlids. In these cases, the easiest solution is to have more aquariums. Another alternative is to use aquarium dividers to keep the peace while spawning and fry-raising is underway.
Growth Rates
It is not uncommon for the same group of fish to have vastly different growth rates. Part of this will be genetic, with a percentage of the group being litter runts. It is partly due to consumption, as more aggressive fish will consistently eat better. It can be due to versatility, any fish which shows a willingness to eat non-typical foods (algae wafers, plant material etc) will have a higher growth rate. Like the litter runts, nature's genetic randomness also provides for a few giants (of a modest size increase). Early diseases can permanently affect growth rates negatively. Internal parasites will cause a significant reduction in growth and possibly death.
Sometimes at more critical juvenile stages, meals will cause rapid jumps
in growth rates, such as with an Oscar consuming a few extra feeders
through luck or extra enthusiasm. With most fish, the difference in
growth rates will not be sufficient for the smaller fish to become a
meal (though Oscars would be an exception), but the smaller fish will
drop lower in the pecking order, and will be most likely to be ostracized
by the rest of the fish. Discus are a good example where an ostracized
fish will not be permitted to eat, and without our intervention, the
fish will eventually succumb to disease and death.
Eventual Size
The easiest to explain, observe and apply, this guideline simply states that if a fish can fit another fish in its mouth, then there is a very good chance that it will. Vegetarian Silver dollars will have no reservations about a high protein snack if they come across a small fish. Plecos are unlikely to eat live fish as they lack the mouth apparatus, but their part in the story occurs when they are swallowed, as they will extend their dorsal and pectoral fins to hook themselves in the mouth of the predator. Unable to swallow or spit the pleco out, the fish will starve. The pleco will sometimes survive the experience, but usually it is left to the hobbyist to extract them manually (to save both fish).
Large and small fish can often reach a happy equilibrium if raised together.
The large fish do not view the smaller ones as food, and the smaller
ones do not exhibit a fear which would excite the larger fish into interest.
The small fish just yield to the larger fish as they swim by. This equilibrium
is most ironically seen when Angelfish are raised with Neon tetras.
In nature Angelfish are carnivores specializing in eating fish (piscivores)
and Neon tetras are the dinnerplate of the Amazon tributaries. These
types of equilibriums are often short lived. You only have to add one
new Angelfish who doesn't know the rules, and soon all the Angelfish
are following the example. Adding a new Neon tetra has a similar result,
either from the Angelfish's natural curiosity in whether the new arrival
is intended to be food, or in the new arrival's frantic reaction to
the Angelfish's advances, making the Angelfish that more aggressive.
Once the new Neon tetra is consumed, it is usually a matter of 2 or
3 days for the other Neons to completely vanish.
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