TPhyllis and Jim wrote, On 19/05/2007 18:48:
... The population of fish (and plants) is self regulating depending on the
available resources.


Gill Passman wrote:
...
The problem with attempting a self-sustaining aquarium is that it is a
closed system whereas nature will take over with a pond. A pond will
attract wildlife that will become a source of food for the fish and
other creatures living there. An aquarium will generally only have what
you have introduced. Because the area is enclosed it becomes very
difficult, if not almost nigh on impossible, to sustain sufficient
foodstuff to maintain without supplemental feeding. In a pond, nature
will supply the foodstuff.

When a breeding population outgrows it's food supply, in a small pond
I would expect it to exterminate it's food supply before suffering
significant losses due to starvation, so it would therefore end up
starving to death. For insectivores and algae eaters that's less of a
problem, because new sources come into the pond all the time. However,
it seems to me that anything which relies on aquatic plants for food
is not going to see its food supply quickly re-established. I had been
hoping to have the fish keep the plants in check, so I wouldn't have
to do any weeding; from what you've said, it sounds like this would be
feasible, and I'd like to understand how that works.

While nature is able keep a small population imbalance in check, I'm
sure it couldn't handle a large one. If I start out with too many fish
and not enough food sources, they will starve. Can you give me any
rules of thumb for how many fish and plants of a given size and type
can be supported naturally by a pond of a given size, and in what
ratios? The rules of thumb I could find for aquaria were based upon
plants artificially supported by pumped CO2, and fish that were being
artificially fed. They assumed that the fish weren't eating the
plants, and that the plants were being trimmed by the aquarium keeper.
As a result, those densities are way too high for the approximately
balanced ecosystem I'd like to set up.