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Re: Aquatic Plants Digest V3 #1399

Roger Miller wrote:

"Commonly recommended change schedules
like 20-25% every two weeks (equivalent to 10% to 12.5% per week) " --snip--
 Not quite.  A 20% change leaves 80% of the original water (100-20% = 80);
but two 10% changes leave 81% (100-10% = 90; 90 -10% =81). Over time this
can make a difference.

Suppose you have 10 ppm of phosphates in you water, and want to reduce it to
1ppm or less. Using just water changes, after 22 10% changes the level would
have dropped to .98 ppm; using 20% changes ever other week would result in
.86ppm.  The 20% changes would give you about 15% less.  In addition, the
average level, over 22 weeks, with 20% changes, would be about 9% less.

John Godbey
Springfield, VA