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Re: prepping aquarium water

"The owner was prepping water
using a 55 gal plastic drum that had been scrubbed and washed. Had a power
head circulating the water and had added whatever chemicals. Said he let it
circulate for 2-3 days, and then hooked hose to the power head to load the
water during water change."

Without knowing what "type" of water he was preparing, its impossible to say
if this was a wasted effort on his part. Was he trying to get something out
of the water (chlorine/chloramine) or was he trying to get something (a salt
mixture) to dissolve into the water? "Whatever chemicals" isn't specific
enough to be able to answer your question.

In general, if dealing with water that contains chlorine (and only
chlorine - not chloramines), allowing it to stand for a few days, with
perhaps some circulation, is sufficient to allow the chlorine gas to
evaporate. But whether or not you need to do that would depend on how much
chlorine was in the water in the first place and how sensitive the animals
in your tank are to chlorine. As far as I know, dechlorinators act quickly
and  if he had added a dechlorinating chemical the water would be ready for
fish almost immediately. Chloramine will not evaporate, of its own accord,
over any reasonable time period - it can persist for weeks so if the water
was treated with chloramine, he would have had to use a chemical (like
Prime) to break up the chlorine-ammonia bonds and then tie up the ammonia.
Again, the reaction is pretty much instant. The ammonia is rendered harmless
but it is still available for uptake by plants or consumption by nitrifying

Maybe there was dissolved gas in the water that he wanted to let
equilibriate with the atmosphere - that could take several hours, maybe a
day. But without knowing the nature of the water, its impossible to say.

If he was preparing water for a specific purpose - say a brackish tank or
for Rift Lake Cichlids, and he had added salts to the tapwater, his mixing
might have been designed to allow all of the salts to dissolve - some can
take a while to fuly go into solution.

"Also, if a planted tank, would not some of the other elements
valuable to a tap water change be evaporated out also?"

No, not likely. The only "nutrient" that is likely to be "lost" on standing
is CO2 - and that assumes that the tap water has more CO2 dissolved in it
than standing water at equilibrium with the atmosphere (a lot of tap waters
do have extra CO2 when freshly drawn, and its easy to determine by measuring
the pH of freshly drawn water and then retesting the same sample 24 hours
later). But the mineral nutrients that would be in tapwater aren't volatile
compounds - they don't evaporate.

James Purchase