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Re: More musing on water change.

At 10:08 AM 7/25/2002 -0700, you wrote:

>Charles n Sue Harrison wrote:
>In response to my:
>>>If poop accumulation is steady, partial changes can never do better than 
>>>reach some ragged equilibrium level of pollution.
>>There is no equilibrium. and that is a common thought, that there must 
>>be.  One adds food, the fish produce waste and evaporation adds to 
>>concentration. Partial changes only remove portions of it. The same food, 
>>fish waste and evaporation continues to build.
>OK, Charles. Can you show me the arithmetic that demonstrates that?

Trying to analyze what is and isn't in tank water, especially in a 
theoretical system, is difficult, but in my opinion, partial water changes 
will produce a situation in which an equilibrium is reached in practical 
terms. Try this:

Let's say you start with a new pristine but cycled tank with no 
contaminants. Let's argue the fish add 100 units of contaminants a week. 
We'll call them "poop units" for convenience. Let's also say you do 50% 
water changes each week, religiously. Here is how the math would go:

Week 1: 100 units added, reduced to 50 units after the water change, then
Week 2: Another 100 added, coming to 150 units, reduced to 75 after water 
Week 3: Another 100 added, coming to 175, reduced to 88
Week 4: 188, to 94
Week 5: 194, to 97
Week 6: 197, 99
Week 7: 199, to 100
Week 8: 200, to 100
Week 9: 200, to 100,
and so on.

The above is slightly inaccurate, because I took the liberty of rounding 
off to the next whole number, and in point of fact, the system may never 
reach absolute equilibrium. However, eventually the system will reach a 
point where changes are infinitesimally small and the water will have a 
certain level of contamination. That level, for any one set-up, may or may 
not be tolerable to the fish.

Furthermore, I recognize that there are many other wastes besides those 
that we measure, namely ammonia, nitrite and nitrates. However, I know of 
no metabolic waste that cannot be broken down by other organisms. There IS 
such a thing as a closed system - we call it earth. It is also true that we 
cannot in practical terms mimic a closed system in a home aquarium. 
However, I do believe that we can partially achieve that. The very fact 
that nitrates accumulate in an inhabited fish tank proves that breakdown of 
nitrogenous wastes is going on. In addition, stable experimental systems 
have been developed in which fish can be maintained for long periods 
without water changes. A lot of research is going on in that area related 
to space research.

Finally, besides harmful stuff, it is likely that beneficial materials 
accumulate in tank water. Edd referred to this in his message, in terms of 
organisms, but there may be soluble compounds that are beneficial too. We 
often hear in advertisements about hormones and so on that are detrimental, 
but I'd be willing to bet there are some that are useful.

My personal take on this is that overall, in a home aquarium situation, 
harmful stuff does accumulate over time.  Therefore, I believe that water 
changes are necessary. However, personally I am not a proponent of 100% 
water changes. What I'd really like to have is a flow through system in 
which fresh water is continually, steadily, being added, flushing out old 
water. (Note that that is not a 100% change system). However, I can't do 
that because I need to modify my water (adding salt, allowing pH to 
stabilize, etc.). Hence, I aim for weekly water changes, in my case about 
33% changes. When I do achieve that, my fish seem to do well.

Well, I have probably added fuel to the fire, but that is my opinion on 
this issue.


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