# Re: the great water change and equilibrium debate

```Hello Doug,

As a first approximation, you can measure a "poop unit" reasonably
accurately for any fish, by measuring the (oven dried) amount (weight)
of food you add to the tank per day or per week.

Divide the amount of food by the number of fish in the tank and you have
the amount of poop per fish. Divide that further by the volume of the
water in the tank and you have the poop/fish/unit volume measure.

Even if the fish don't eat all the food you add, it will be there to
polute the water.

In growing fish some of the food will end up in body weight and thus not
add to the polution, but that will be a safety factor in the right
direction, i.e. indicating a bit more poop than is actually present in
the water.

One can follow the buidup of known amounts of poop and measure, say,
conductivity (TDS) and construct a graph of poop vs. conductivity. Then
only conductivety (at constant volume) needs to be measured to estimate
the amount of poop present in the water.

What will we call the science (art?) of estimating poop -- "poopology"?
:-)

Best,

George

Doug Dame wrote:
>
> I think perhaps we lay-people are using the term "equilibrium" in a casual way, whereas the scientists naturally tend to hear and use the term per it's technical definition.
>
> A change in semantics could be helpful
>
> What we are really talking about is not a steady-state equilibrium per se, it's more akin to a "control range" in the world of statistical process control (SPC) ... where we establish an "acceptable" upper and lower limit on the thing we're can about ( was it Barry who suggested poop unit per liter?) , and then try to make sure that our [water maintenance] process can keep our water quality within the target limits.
>
> Unlike classic SPC, in the world of killie-keeping the upper and lower target levels must be established by judgement rather than empirically by statistics, and is influenced by many factors ... toughness and nature of the fish, size of the tank, etc. etc etc.
>
> Say I wanted to keep my tanks between the levels of 5 and 10 poop units per liter. (That's because, with no good scientific basis whatsoever, I think 15 PUPL is where bad things happen. I want some extra margin of safety for when I go out of town for a week or an apple snail dies out of sight without giving proper notice.)
>
> Is there an approximate formula for estimating the daily poop unit production of fish of various sizes ?  If I could estimate the rate at which poop unit are produced, I could back into the amount and frequency of required water changes.
>
> Some people may have fish that they believe enjoy and benefit from a near-total refreshing of their water. In that circumstance, a control range of 1 to 25 PUPL may be the goal. Again, given that a specific goal, and some estimate of the rate of poop unit accumulation, a reasonable water change schedule could be determined.
>
> Here's a question (Wright?) .... can we use change in TDS as a proxy for accumulated poop units and water quality ?  (And extremists could empiricaly manage their water quality with SPC control charts on TDS ???)
>
> Inquiring Minds Want to Know.
>
> Doug Dame, SKS
> Interlachen FL
>
>
>
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