# 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
change
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
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
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.

Barry

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