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Re: why do water changes

> I know that we are supposed to do regular water changes (I do mine weekly)
> even with planted tanks.  My question is why?  I'm not doubting they are
> necessary, its just that I'm having a hard time articulating exactly what is
> accomplished by doing them.  Essentially my reasoning usually comes down to
> something like "to remove bad stuff", hardly scientific.

How about to keep nutrients from getting too high if they are not being used
and dosing to make sure they don't run out?

This way you can keep things in a good middle ranges for the plants.
> With a non-planted tank it is is much more obvious to me why we do it since
> there is nothing to pull waste products such as N, P, K, etc. from the water.

Well they do have denitritfiers etc.
> But since plants uptake the minerals in the waste products (especially
> nitrogenous waste, the main element of concern for fish), just what are we
> doing by changing the water?

See above.
But you are also removing mulm and other organic particulate matter which
the breakdown into dissolved organic waste(N, P, K, traces etc) consumes O2.
You also scrap off and remove algae and their associated spores are removed
after a pruning/trim followed by a water change. Often after a pruning you
can see the O2 levels drop way down.
This drop in O2 often favors the algae, not the plants.

If I need to make an anaerobic sample in the field, I take some substrate
from a lake etc, add to the bottle, shake good, and after a few minutes the
bottle has zero O2. The bacteria and all the matter is exposed to the O2 and
quickly consumes it. You can measure this with a Redox meter or an O2 meter.
The Redox meter will give you a better reading IMO.

Water changes are a good disturbance that beat on the algae and help the
plants(adds lots of CO2/O2 and good circulation etc). More O2 is generally a
much better thing for the plants vs the algae.

Tom Barr