I do 20% water changes bi-weekly, have for four years now.
The water change is an important tool for a newbie. It can be an immediate source of aid in some cases.
The last article Mitch linked to specifically mentions water changes as necessary to correct ionic imbalances that cannot be corrected otherwise.
When messing with Ca and Alk, many newbies tend to start adding one sided supplements and throw off their Ca/Alk balance badly. Water changes are the way to remedy this and then proper supplementation can begin. More can be found in Craig Bingman's articles.
I will also quote a comment he (Craig Bingman) made (googlegroups):
"There are still some old hands who pride themself in having never done a
water exchange on their systems, or for having gone N years without
having done a water exchange. I've looked at the chemistry of some of
these systems. While some of them are OK, others are way out of whack.
The usual response to that is that "stability must be more important than
aabsolute concentrations." To a degree, there is truth to that, but
given advances in water purification technology, salt mixes, etc, there
isn't any real reason why doing a water exchange should be destabilizing
anymore. Doing a partial on these systems that have badly drifted over
time might be a bit of a shock to the residents, but man, it must be one
*hell* of a shock for an organism coming into that system. And if a
series of modest partial water exchanges had been done over time, the
system would have been stable, and it wouldn't have drifted that far from
normal seawater chemistry."
--end quote
IMO, the water change is one of the VERY most important aspects to be aware of, whether you keep it in your back pocket for when you feel you need it, or look at it as preventative maintenance. No two reefing situations are identical.
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