Both in terms of fish size, and in terms of stocking levels. It is clear to me that there is no single rule that is both necessary and sufficient for all cases, and because of this I think it would be good to discuss which rules apply in which kinds of situations.
First, the one inch per gallon rule: the rule of thumb.
This used to be measured as inches of fish in terms of length in comparison to total # of gallons in the tank. Now I believe that it has been refined into a more complex calculation involving the inches[SUP]3 [/SUP]of fish volume and the surface area of the tank, or something like that. In any case, it may work for what it is designed to be, a rule of thumb, but there are many exceptions to it, and it only works (approximately, at that) in a certain range of tank sizes/shapes.
Maybe the primary (or theoretical) intent of the inch per gallon rule is to prevent stocking levels that would result in there not being enough dissolved oxygen for the fish in the tank to breathe well. My reason for saying this is that the method of calculating capacity as a relationship between fish size and tank size can (in theory) account for the factor of oxygen depletion can at least be somewhat accurate, most of the time (excluding air-breathers, and assuming normal levels of gas exchange). Perhaps it can account for waste/bioload to a certain extent, but this depends even more on the type of fish, I believe. Therefore...
1. The tank (surface area) must be able to provide all it's inhabitants with sufficient dissolved oxygen to breathe properly.
These two criteria, if they are calculated accurately, are necessary conditions for any tank. Thus, we can be fairly objective about their use; in fact, if these criteria are not met, your fish will probably become visibly ill and/or die pretty quickly, and there will be no doubting that your tank was too small for the stocking levels.
However, they are not sufficient conditions for many tanks. They might be good enough for fish below a certain size threshold that are peaceful, non-territorial, and have no special needs that complicate things, but outside of a small community setting we need more guidelines. This is were the implementation of rules becomes increasingly subjective.
I will continue with a tentative list of further criteria in the second post.
First, the one inch per gallon rule: the rule of thumb.
This used to be measured as inches of fish in terms of length in comparison to total # of gallons in the tank. Now I believe that it has been refined into a more complex calculation involving the inches[SUP]3 [/SUP]of fish volume and the surface area of the tank, or something like that. In any case, it may work for what it is designed to be, a rule of thumb, but there are many exceptions to it, and it only works (approximately, at that) in a certain range of tank sizes/shapes.
Maybe the primary (or theoretical) intent of the inch per gallon rule is to prevent stocking levels that would result in there not being enough dissolved oxygen for the fish in the tank to breathe well. My reason for saying this is that the method of calculating capacity as a relationship between fish size and tank size can (in theory) account for the factor of oxygen depletion can at least be somewhat accurate, most of the time (excluding air-breathers, and assuming normal levels of gas exchange). Perhaps it can account for waste/bioload to a certain extent, but this depends even more on the type of fish, I believe. Therefore...
1. The tank (surface area) must be able to provide all it's inhabitants with sufficient dissolved oxygen to breathe properly.
Variables: air-breathers, turbulence levels, water temperature, activity level and metabolism of inhabitants, fish size (I am presuming that big fish consume more oxygen per inch[SUP]3[/SUP] than small fish), etc.
2. The tank (gallonage) must be large enough for the bioload and nitrate levels to be manageable.Variables: eating habits and metabolism of inhabitants, uneaten food consumed by inverts or not, volume of plant mass, filtration, and fish size.
These two criteria, if they are calculated accurately, are necessary conditions for any tank. Thus, we can be fairly objective about their use; in fact, if these criteria are not met, your fish will probably become visibly ill and/or die pretty quickly, and there will be no doubting that your tank was too small for the stocking levels.
However, they are not sufficient conditions for many tanks. They might be good enough for fish below a certain size threshold that are peaceful, non-territorial, and have no special needs that complicate things, but outside of a small community setting we need more guidelines. This is were the implementation of rules becomes increasingly subjective.
I will continue with a tentative list of further criteria in the second post.
