# Re: More musing on water change.

```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
composition.
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
slowly.

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 the undissolved 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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