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cellar chemistry #1

As I said in a previous post, I never worried much about tap-water quality
until it was a problem.  Now that I have studied it and understand it
better, I worry about it a lot!  [ignorance is bliss?!]   At first, I was
using RO water just as a base to reconstruct tap water.  But after I went to
great extremes to get the initial concentrations of all the ions just right,
evaporation would go and mess it all up by changing the concentration of
water around my ions!  Should I then be topping off with RO water before I
do water changes, to bring all those ions back to their original
concentrations??  Evaporating water presumably leaves its dissolved contents
behind, concentrating the water in the tank.  Adding pure RO water would
seemingly bring it back to its original concentration, but adding
reconstituted water might make it more concentrated with solutes (harder,
saltier) with every top-off.  The purist in me had to know how significant
this factor really was.  [yes, I have way too much spare time]  I set up the
following basement experiment:

Three 5-gallon buckets were filled with tap water and churned with an
airstone.  When each had lost 1 gallon (20%) over time to evaporation, they
got one of three 'water change' treatments.

- bucket 1 represents the lazy aquarist; the evaporated water was simply
made up for with straight tap water and no water was changed.
- bucket 2 represents the avaerage aquarist; after 1 gallon was lost, a
second gallon was siphoned out and the 2 were replaced with straight tap.
- bucket 3 represents the anal aquarist; the evaporated gallon was replaced
by an RO gallon before a 1 gallon water change using straight tap.

'Concentration' was measured with a PinPoint conductivity meter.
Measurements were taken 1 day after water changes, as conductivity rose
significantly over the first 24 hrs.  [I don't know why it would, perhaps
dissolved gases have some effect, but it did stabilize after a day, so I
took all measurements only after 24 hrs]

It's been 1 month and I just did the 3rd water change.  Here's the deal thus

date     bucket 1     bucket 2     bucket 3      tap
5/2       start
5/3       215             215            215             215
5/13     fan added for forced convection (I got impatient!)
5/16     water change #1
5/17     261             251            218             221
5/24     water change #2
5/25     279             259            207             227
6/1       water change #3
6/2       300             275             205            227

In just 1 month (granted, with forced convection), the lazy aquarist had
raised conductivity by 40%.  The average aquarist raised conductivity by
28%.  The anal aquarist actually lowered conductivity (I'm guessing that's
explained by the 'salt' stains above the water line).

Interesting.  Probably not as much an issue for the keeper of planted tanks,
where presumably many of those extra ions are used by the plants.  But it's
food for thought/discussion.
Kevin [really bored in Detroit] Z.