The answer is in bacterial reproduction.
At mid twenties Celsius temperatures the bacteria thrive, and reproduction
exceeds attrition, so that every available surface in the tank gets covered
in bacteria.
Apparently the bacteria suffer the same fate as all of us nowadays, early
burn-out, and forced redundancy at a youthful age. So the reproduction needs
to be high to replace those who shuffle off.
But when the temperature drops out of the twenties reproduction reduces
dramatically, attrition is ahead of reproduction, until it all settles to a
much lower sustained balance.
So the reason that biological filters don't function as well in cooler water
is simply that there are far less beasties on the job, in both senses of the
term.
It may well be as you suggested, that the higher oxygen level will allow the
bacteria to function more efficiently, but with less of then at it, trying
to maintain the same fish stocking levels as in a tank at summer
temperatures, can lead to problems.
Doug Collom
-----Original Message-----
From: Doug Collom <dcollom at powerup.com.au>
To: rainbowfish at pcug.org.au <rainbowfish at pcug.org.au>
Date: Saturday, 14 November 1998 23:10
Subject: Re: [RML] Designer Fish
>Yes Bruce you are right, we would have to ask more learned heads for expert
>advice on this one.