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need for partial water changes
Kevin Buckley asked why we need partial water changes in a properly
stocked, filtered, monitored and PMDD-fed planted tank.
The most evident reason is that we aren't perfect and the water
changes give us a fall back that keeps problems from accumulating.
In particular, fish and other animals excrete a large number of metabolic
and digestive wastes aside from those that we monitor. Most of those are
probably in small quantities and may be broken down in a biological
filter. If they aren't broken down or if they are broken down to
something more stable (and worse case, more toxic) then those byproducts
will accumulate in the tank.
With regard to the use of PMDD, keep in mind that recipe is intended to
add all things except phosphorus *in excess* of the plants' needs to make
sure that phoshorus will be limiting. That means that all of the minor
and trace elements that can't be measured and adjusted in the recipe will
tend to build up. Some of the usual trace constituents (Cu, Zn, B, and
others) are toxic if they accumulate to sufficiently high levels. Water
changes are the only way to prevent their long-term build up.