Bleeding heart tetras Hyphessobrycon sp.; the
actual water quality in a tank is a function of
both how often water changes are conducted
and how much water is changed each time.
In Part 1 of this article (TFH November 2009) we stablished that while small daily changes maintain a more consistent water quality, a weekly water change of the same total amount provides your fish with much higher water
quality, with significantly lower pollution levels for much of the
time. In this conclusion we will investigate the actual water quality
maintained by changes of various percentages.
How Much Is Enough?
Another motivator for my research was the common advice to
change amounts like 10 or 20 percent weekly or biweekly to keep
water quality high. You will also come across the counterargument
that such small changes are hardly better than none. Which is true?
Again, the calculations might amaze you. Let’s look at a set of water
change regimens between the extremes we looked at above, in
The shape of the graphs of regimens of 10, 20, 30 percent, etc.,
is the same, with amount of dissolved crud rising for six days to
a maximum, plunging back to a minimum after the weekly water
change, and repeating the cycle. What differs greatly is the time
it takes to reach equilibrium (asymptote) and the minimum and
maximum values for the amount of pollution in any given tank. It
is easier to compare these data in tabular form rather than looking
at 10 graphs simultaneously.
Consider Table 1; these minimum and maximum figures indicate
the pollution levels at asymptosis in terms of days—they indicate
what these levels would be in a particular aquarium set up and
maintained with no water changes for that number of days. Thus,
a 20-percent weekly change, which many people consider a very
good regimen, will indefinitely maintain a tank at pollution levels of
about a month. It will always be as polluted as if it were set up and
run for a month with no water changes. Another way of looking at
this is that the tank will always have a pollution level equivalent to
a month’s accumulation of the daily waste production. Of course,
some people recommend only a 10-percent weekly change.
Even after 242 days, the 10-percent weekly tank fails to reach
asymptote, so the maximum pollution is something more than 67
days’ worth, and the minimum pollution is something more than
62 days’ worth. In other words, with a 10-percent weekly water
change, the amount of crud in the aquarium will always be greater
than if you left the aquarium with no water changes for more than
nine weeks. And yet some people talk about such a water change