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Re: More musing on water change(sent earlier, but did not get through)

I have been sitting on an article for JAKA that deals with this subject. It 
uses mathematical models and some simplified assumptions to help provide 
guidance. In summary, it states that it all depends on the assumptions you 
make and what you use to replace the changed water. If the water deteriorates 
at some level (say 10% between water changes) and you use the original water 
supply as replacement, then a steady state can be achieved that is slightly 
higher in pollutants than the tap water. If you dilute the tapwater with RO 
water (assumed to be zero contaminants), then you can use less percentage 
water change to achieve the steady state. If, on the other hand, you assume a 
more significant deterioration and a smaller water change, the water gets 
progressively worse in quality. Then you get in the situation that we want to 
avoid -- that is the water is now significantly different from the 
replacement water and a massive water change will create a shock to the fish. 
Then you have to do a lot of smaller changes to get back to the desired 
state. In any case, the more changes you do, the better you maintain the 
lower level of contaminants, assuming the replacement water has an acceptable 
    An example that is probably illustrative of this is the following:
Assume that the tap water has 400 ppm TDS and you use it for the initial and 
replacement water. Also assume that the water deteriorates at 10% added TDS 
between water changes. Then the water reaches a steady state after 4 water 
changes of 50%, where the value goes to 488 ppm and returns to 444 ppm at 
each water change, Probably an acceptable state for most fishes. If you need 
lower TDS then you start with that and use that for replacement.
    However if you do only 20% water changes with the above scenario, the 
value will climb to 665 ppm after 40 water changes and is still climbing 

    Big caveat -- the above is totally focussed on one parameter. In real 
life, upsets in aquaria occur, like overfeeding, a fish dies, some plants 
start flourishing, etc.
These make these basic principles just that -- basic principles, not hard and 
fast rules.
    In any case, more frequent water changes are better than infrequent water 
changes. Higher percentage water changed is better as long as the change is 
not too drastic and the water quality going in is OK. Diluting the source 
water with DI or RO water helps dilute any dissolved materials that are 
already present in the tap water or to maintain a lower level of TDS, for 
example, than the tap water provides.
    None of this addresses the issue of t sediments, which can be beneficial 
or harmful, but in any case may be unsightly.
    Another factor that this model reveals is that you can only decide what 
is the proper amount of water you should change if you know the amount that 
contaminants are increasing between water changes. One over simplified way to 
do this is to check some easily measured parameter, like TDS. Then you can 
choose your water changing methods to control that rate of change.

Lee Harper
Media, PA
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