Inches of fish/gallon guidlines = NONSENSE!

[Don]

I have frequently run across guidelines for the number of fish that
can thrive in a given size aquarium expressed in inches of fish per
gallon. I trust the following assumptions are reasonable:

*Oxygen consumed and waste produced are roughly equivalent to body
mass

*Oxygen consumption and waste production are the primary factors in
determining minimum healthy space requirements for fish.

*The proportions of a fish are similar regardless if the fish is one
inch long or ten inches long.

OK. Body mass is equivalent to the 3rd power of the fish's length.
So 10 fish 1 inch long have only 1/100th the total body mass of 1 fish
10 inches long. Yet according to the frequently quoted rule of thumb
they have the same space requirements! Let's use a little less
extreme example: 10 fish 1 inch long = 10 arbitrary units of mass. 5
fish 2 inches long = 40 of the same units! So we a have discrepancy
of 4/1! That's still way more than enough to render the rule useless.

Is there something I am missing? Seems to me we need to add up the
length of each individual fish taken to the 3rd power and only then do
we have a usable guideline.

Don

[Roger Sleet]

In article >,
(Don) wrote:

> *From:* Don >
> *Date:* Sat, 13 May 2006 12:10:19 -0500
>
> I have frequently run across guidelines for the number of fish that
> can thrive in a given size aquarium expressed in inches of fish per
> gallon. I trust the following assumptions are reasonable:
>
> *Oxygen consumed and waste produced are roughly equivalent to body
> mass
>
> *Oxygen consumption and waste production are the primary factors in
> determining minimum healthy space requirements for fish.
>
> *The proportions of a fish are similar regardless if the fish is one
> inch long or ten inches long.
>
> OK. Body mass is equivalent to the 3rd power of the fish's length.
> So 10 fish 1 inch long have only 1/100th the total body mass of 1 fish
> 10 inches long. Yet according to the frequently quoted rule of thumb
> they have the same space requirements! Let's use a little less
> extreme example: 10 fish 1 inch long = 10 arbitrary units of mass. 5
> fish 2 inches long = 40 of the same units! So we a have discrepancy
> of 4/1! That's still way more than enough to render the rule useless.
>
> Is there something I am missing? Seems to me we need to add up the
> length of each individual fish taken to the 3rd power and only then do
> we have a usable guideline.

Well it isn't a rule it is a guideline i.e. it isn't an absolute. The
fact that it appears in both UK and US literature makes that clear - US
tank can be more crowded than UK tanks doesn't really hold up. Things
like the shape of the tank are also important. A tall thin tank can hold
less fish than a short broad one of the same capacity. As a result I
prefer the other definition of 1" of fish for each 12"sq of surface area.

All your assumptions are wrong anyway.

> *Oxygen consumed and waste produced are roughly equivalent to body
> mass

Might be true of warm blooded animals, but not of fish. Activity is far
more important, a shoal of Danios will use much more oxygen than a similar
weight of Angelfish.

> *Oxygen consumption and waste production are the primary factors in
> determining minimum healthy space requirements for fish.

No you can increase the ability of the system to handle both by varying
the amount and quality of the filtration. Ability to turn round and
sufficient depth of water are pretty critical too.

> *The proportions of a fish are similar regardless if the fish is one
> inch long or ten inches long.

Only to a limited extent within a species. Clearly they aren't the same
when comparing a Stocky cichlid with an Eel. Even within a species it
isn't true, a lot of fish change shape considerably when they become
sexually mature.

The main point is it works OK for a normal mix of community tropicals.
Obviously putting a fully grown Jaguar cichlid in a 12 gallon tank would
be silly, it could barely turn round, however 12" of normal community fish
would be fine.

Roger Sleet
Roger's Aquatic Pages http://www.sleet.plus.com