Even the best filtration cannot remove all the dissolved pollution in aquarium water—only water changes can.
Most aquarium maintenance isamatter of crud and how to get rid of it. A mechanical filter traps suspended crud, a chemical filter emoves certain dissolved crud, and a biofilter
changes very toxic dissolved crud into much less toxic
dissolved crud. But no matter how much filtration you have
on your aquarium, dissolved crud in some form continues to
We don’t even know everything that accumulates, but we
do know that keeping the amount of dissolved crud to a
minimum really makes a difference in the growth, health,
and apparent happiness of our pet fish. The simplest, easiest,
and most effective way of removing the accumulated pollution
in an aquarium is to do just that—physically remove it by
taking out the water and replacing it with fresh, clean water.
The importance of water changes led me to mathematically
investigate them, and the results will probably surprise you,
since they argue against some common wisdom in the hobby.
A Numbers Game
Aquarists make a lot of claims about water changes, but they
hardly ever offer anything other than anecdotal evidence, if that.
The nature of water changes, however, lends itself to simple
arithmetical explanations—you take a certain percentage out; you
replace it. Don’t worry, the math here is very simple, and I’ve done
all the work for you.
Every fraction of the total volume that you remove contains the
same fraction of the total dissolved pollution—change half the water,
you remove half the dissolved crud. That part is easy to understand.
What many people fail to do is consider the pollution that remains
in the aquarium. They perform the next water change, thinking
that they are starting over again, removing a certain percentage of
the pollution. They forget that some of the original crud—usually
most of it—from before the first water change is still there, along
with a whole new accumulation of crud. Every water change brings
them a good feeling—after all, they’re removing 20 percent of the