# How many fish can go in my tank?



## Chris S (Dec 19, 2007)

Inch per fish rule? Size limitations? post your thoughts.


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## KnaveTO (May 13, 2007)

The inch per fish rule is extremely outdated. The most accurate I have seen to date is the following

Fish Standard Adult Length is 1-5cm then 1cm of fish per litre
Fish Standard Adult Length is 6-10cm then 1cm of fish per 2 litres
Fish Standard Adult Length is 10+cm then 1cm of fish per 4 litres

Standard Length does not include the tail in measuring the fish length


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## Shattered (Feb 13, 2008)

KnaveTO said:


> The inch per fish rule is extremely outdated. The most accurate I have seen to date is the following
> 
> Fish Standard Adult Length is 1-5cm then 1cm of fish per litre
> Fish Standard Adult Length is 6-10cm then 1cm of fish per 2 litres
> ...


For those on the imperial system: 1 g. us = 3.785 L

and:

cm x 0.39* = in
in x 2.54 = cm


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## gunnerx (Mar 20, 2008)

Am I allowed to steal from another forum? I find this to explain the One Inch per Gallon rule more explicitly.



> This is something that comes up fairly often and is rather misunderstood therefore I will attempt to clarify the original meaning of the general guideline of "one inch of fish per gallon of water".
> 
> This is a suggested guideline for a well maintained and filtered tank.
> It does not apply to all fish as some have differing requirements.
> ...


Source


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## Chris S (Dec 19, 2007)

No problem taking information from different areas, just make sure you cite the site and the author - give credit where credit is due.

Good little article though.


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## KnaveTO (May 13, 2007)

Figured I would do a bit of calulating

Using the cm/litre rule the teo fish quoted above would use the following

Gourami = 48 litres of water or 12.7 gallons
Tetra = 4 litres of water of 1 gallon

Under either rule bioload can be maintained with a higher density of fish so long as a active and extensive water change regimen is in place. I would be interested in finding out what the advised water regimen is for both of those rules. I use a 20-30% weekly water change and maintain a planted tank so could handle a higher density than someone only doing a 10-15% water change weekly with plastic plants.

That is the catch to those rules. Outside factors of plant density and water change frequency warp them one way or the other


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## kweenshaker (Mar 11, 2006)

i'm confused lol


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## dekstr (Oct 23, 2007)

I think the inch per fish rule has been over simplified to help explain the stocking levels, AFAIK for FW aquariums.

Even the fish volume / water volume rule of thumb is over-simplified. There are many other factors involved in stocking levels that are left out.

For example, these rules leave out fish requirements for swimming space, fish behaviour, frequency and size of water changes, filtration capacity/power, frequency of filter maintenance and plant density (if applicable).

*Swimming space: *Different fish need different amounts of swimming space. Most schooling fish require long tanks in order for them to swim naturally side to side. As well, you need multiple numbers of the same species of schooling fish in order to make them behave more naturally.

*Fish behaviour*: Does the fish prefer to swim in certain parts of the aquarium? Is it territorial and mark out its own area to defend? Can it coexist with other tankmates (if a community tank)?

Even if you can fit a certain number of fish according to the fish per ___ rule, will there be enough room for them to swim normally? For example, you may fit 2 schools of swimming fish in an aquarium, but if they swim in the same level/area of the tank, they may frequently collide and get in each other's way before they can get a good swimming pattern going.

On the other hand, if you keep fish that swim on different levels of the tank, even with the same # of fish, they will have their own areas/levels to swim along in.

You also have to consider that some fish may be territorial. Even if there is enough water volume for them, they may attack other fish to defend their own territory.

*Frequency and size of water changes*: You can keep the water clean depending on the frequency and size of water changes you do. The same number of fish that may seem overstocked in one aquarium may be kept very clean if frequent water changes of appropriate sizes are done.

Of course it is safer to not push the limits of stocking levels in event you slip up on the water change schedule. As an extreme example, you may be able to fit 50 guppies in a 20 gallon tank if you did massive water changes three times a day, but it is not very practical.

Filtration capacity and power: With a more powerful filter, it will still be very efficient for longer periods between cleaning before it becomes clogged and less effective. This is why it is useful to use a filter that is rated over the volume of your tank. The listed gallon per hour rating of the filter by the manufacturer is the 'ideal' number. This is before waste and debris and filter media slow down the flow of the filter.

Of course there are limits to overfiltration. The bacteria in the nitrogen cycle can only process so much ammonia and nitrite so fast. Even with a very powerful and efficient filter, there can be more waste than what the filter can process.

Frequency of filter maintenance: A filter becomes less efficient as it accumulates more waste. Even powerful filter will be useless if it is clogged.

Plant density: Aquatic plants have a unique advantage in that they can serve as additional biological filtration in an aquarium. They prefer to uptake ammonia, nitrite, then nitrate as a source of nitrogen, and also uptake many other nutrients into their structure, some of which may be harmful in high concentrations. In fact, nitrate is often supplemented in a planted tank. As well, during the day, plants take in CO2 and release oxygen into the water--a bonus for fish. So with a fast-growing plants and depending on the density of plants in the aquarium, you get the added benefit of extra biological filtration to help keep the aquarium that much better for fish.


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