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Re: More musing on water change.

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?

If the added poop/concentration each week is *less* than that contained 
in 25% of the total volume, you removed more than was added, with a 25% 
change. How does that eventually reach a solid sludge stage? Sounds like 
a classic partial dilution to me.

Equilibrium (and no further increase in concentration) happens when the 
amount of waste removed at each change is just as great as that added 
between changes. That means the tank is certainly not pure, but it can 
mean the quantity of polluting elements can be safely below any harmful 
threshold. [Waste food and evaporation are taken care of by deliberate 
vacuuming/scavengers and adding a bit more water than was present at the 
start of the change.]

Tom's recent post about his flow-through system is a classic case of 
partial changes (sump only) doing an excellent job. The filtration takes 
care of much of the rest and his baby-fish growth proves it is working.

> There is only one way to get back to the start . . . Change it all.

Our difference, I think, is in definition of "start." If you mean zero 
nitrates and all other bad stuff (not already in the tap water), then I 
agree 100% with you.

OTOH, if I mean safe level of nitrogenous products that are below the 
damage threshold, the practical experience of thousands of aquarists 
(and the entire aquaculture industry) demonstrates that partial changes 
do work just fine. [They just *must* do them in all but the most 
overplanted/underpopulated huge tank.]

> "Change as much water as often as you can."

Oleg Kiselev once pointed out "There's no such thing as too many water 
changes." You would add that "There's no such thing as changing too much 

I agree heartily with both of you, but started this new version of the 
thread to point out that partial changes *can* work and often have 
practical advantages for many fishkeepers with less than a PhD in chemistry.

I tend to change way more than 50% of the water in small baby containers 
and nearly 100% on eggs. The hastle of matched, treated water is less 
there. OTOH, my bedroom-fishroom has no room for a sump/holding-barrel, 
and my tap water is lethal as delivered, so I'll keep doing partial 
changes by bucket until I can work out a better arrangement. The fish 
are growing well and giving me eggs when mature, so that has to satisfy 
me until I buy a property with a flowing spring. :-)

BTW, there are no water meters here, and water is a flat monthly fee! 
What a great place to experiment with a constant-flow water-change 
system! Just wait until I get my carbon filters and RO unit hooked up 
and I'll do some experiments. :-)

Meanwhile, off to do my Sec. stuff for BAKA. [The water changes will 
have to wait as I still need to post the minutes of last Sat. meeting.]


"Deport all the product-liability lawyers to Iraq;"
                                 -- Dave Barry --
       [Best suggestion of 2002, by far...]

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