Figure 5: A comparison of the amount of accumulated dissolved solids
between a tank that receives 10-percent daily water changes versus one
that receives 70-percent weekly water changes.
10% Daily - 70% Weekly
to endure a third of the pollution the fish in the other tank endure
every single day.
It would take a series of complex and carefully structured
experiments to determine empirically whether fish are more harmed
by a constant level of pollution or by varying pollution levels.
Intuitively, it seems that overall exposure is more significant. After
all, what would be your choice if you could choose between living
in a town with a consistently high level of air pollution and smog
and living in a town where once a week pollution and smog reach
that same high level, but then it drops to one-third that level and
gradually rises back to the maximum over the rest of the week?
Even the deadliest poisons are relatively harmless at a certain
low concentration. As the concentration increases, so does the
11 It is therefore reasonable to conclude that a regimen
which provides pollution concentration levels that are frequently
lower than those with another regimen will be less harmful.
There is nothing to suggest that living things can get used to
poisons in their environment, making a steady concentration of
a toxin preferable to a cyclical concentration that often is below
the steady one. A large weekly change (as against smaller daily
changes) exposes your fish to a considerable amount of time at
lower pollution concentrations and minimizes the time they spend
at the highest concentration.
What about the argument that large water changes
stress fish by changing water chemistry (other than what
pertains to pollution) too quickly? There does seem to
be some evidence that a massive difference between the
parameters of the tank water and of the replacement
water can shock fish. The underlying assumption here is
that metabolic processes and evaporation change the
chemistry of the water so that over time it becomes vastly
different from that of your tap water. A simple solution is
not to let that happen!
Why wait until water quality degrades to do something
about it? If you change water often enough and in
sufficient volumes, there will be no major changes in its
chemistry, and the new water won’t stress your fish at all.
Often the claim is made that small daily changes are preferable to large weekly ones because they provide a consistently higher water quality. This claim is demonstrably false. A regimen of 10 percent daily changes does produce a consistent water quality, but it is a much poorer water quality than a 70 percent weekly change produces after every change and for several days thereafter.
We’ve seen how contrary to much advice large weekly changes
provide a better environment for your fish than do small daily
changes when the total amount of water changed is the same (here
10 percent daily and 70 percent weekly). In Part 2 we will challenge
common wisdom about the benefits of different total percentages
of water change and reveal more startling discoveries of this
1. Removing 20 percent of the water removes 20 percent of the total
dissolved pollution, which total is made up of 80 percent of the first
week’s leftover pollution and 100 percent of the second’s additional
pollution, leaving 0.80( 80) = 64 percent of first week and 0.80( 100)
= 80 percent of the second week. Alternatively, you could say that
of the 180 percent of one week’s pollution still in the tank before
the second water change, 144 percent, or 0.80(180), remains in the
aquarium after the second water change.
2. Appropriately pronounced pee-yoo, pyoo, or even poo.
3. If the biofilter is immature or inadequate, then ammonia and/or
nitrite will also be part of the total dissolved pollution.
4. Or, at the end of the first day there are x pollution units dissolved in
the water. At the end of the second day there are 2x, the third 3x, etc.
If on the seventh day (accumulation 7x) half the water is removed
and replaced, half of the pollution ( 3.5x) is also removed, leaving
3.5x in the tank.
5. Calculations for Figure 1: The data for this graph are generated by
the simple equation y = x + 10 for x > 0. Or using a spreadsheet, the
formula for B1, starting with A1 at a value of 0, is =SUM(A1, 10), and
continuing on across.
6. Evaporation increases the concentration even further, of course,
but to keep things simple we will assume a very tight-fitting
lid and minimal evaporation. In fact, replacing evaporation with
tap water actually increases the concentration of some dissolved
substances—further motivation to change water rather than to top
7. Calculations for Figure 2: With A1 at 0, the formula for B1 is
=SUM(A1, 10), continuing for six cells. The value for G1 is 0, with
the cycle repeating every seven days.
8. Calculations for Figure 3: A1= 10, B1 =(SUM(A1, 10)) *0.9, continuing
9. The exact value at which the graph reaches asymptote depends on
how many decimal places you carry in your calculations. The more
places you use, the more water changes it will take to reach a steady
value for the remaining pollution at a given rate of water changing.
In fact, if you use infinite decimal places, the graph will never reach
asymptote. After each change there will be an infinitesimally greater
amount of pollution remaining. However, the difference between 1.0
x 10-15 pu and 2.0 x 10-15 pu can hardly be of consequence. In fact,
carrying any decimal places at all is certainly inaccurate, since you
will not be changing precisely the same amount of water each time.
Change a few more drops than last time, and the remaining dissolved
pollution will actually be less than it was after the last change. This
is for the most part way beyond our ability to measure and certainly
a fish’s ability to perceive. Buckets, not pipettes!
10. Calculations for Figure 4: The formula for B1 is =SUM(A1, 10),
continuing for six cells. The value for G1 is =(SUM(F1, 10))*0.3, with
the cycle repeating every 7 days.
11. Toxicity is typically defined in terms of the LC50, the concentration
at which 50 percent of victims exposed die. D
Tropical Fish Hobbyist www.tfhmagazine.com