Calculating PU Thus, for this argument we are assuming that in a given aquarium all feeding and metabolic processes produce a certain amount of pollution each day, which we call—for ease of calculation— 10 pollution units. This means that when the aquarium is set up there are 0 pu in the water. At the end of the first day there are 10 pu dissolved in the water. At the end of the second day there are 20 pu, the third 30 pu, etc. If on the seventh day (total accumulation 70 pu) half the water is removed and replaced, half of the pollution ( 35 pu) is also removed, leaving 35 pu in the tank.
4 The least water changing regimen is zero, no water changes at all. In this case the dissolved crud doesn’t go anywhere, and it just keeps accumulating and accumulating. If we graph this concentration in terms of 10 pu per day, we get a graph like Figure 1.
The urgency of the need for water changes depends on various factors,
including stocking level and the sensitivities of the species included.
feeding, and activity levels will produce relatively little pollution
or a great deal. We also know that certain fish produce more crud
than others. But none of this affects our calculations—it only affects
how we interpret them to determine what water change regimen
we should be using. In an overcrowded tank with lots of waste
production, we’ll want to choose a regimen that leaves a very small
percentage of crud, while we might be more lax with an understocked
aquarium whose inhabitants produce very little waste.
Figure 1: The amount of accumulated dissolved solids in a tank with no
There are no surprises here; the pollution just increases at a
steady daily pace. This particular graph goes out for 70 days, and of
course the pollution reaches 700 pu.
What does the graph of the other extreme, the maximum of 100
percent weekly water changes look like? See Figure 2.
The beauty of water changes is that they remove absolutely all
dissolved crud. Many aquarists test the nitrate concentration in their
tanks to determine if they should do a water change. While the steady
buildup of nitrate certainly indicates declining water quality, there are
a host of other substances in the dissolved crud. A chemical filter that
removes nitrate would lower the concentration of nitrate in the water,
but all the rest of the crud would remain. Live rock, a plenum, or any
other denitrifier would also remove nitrate, but nitrate only.
If you perform sufficient water changes, you won’t have a buildup
of nitrate or of any other substances that compose the dissolved
crud. We are concerned with total pollution, not with the details of
exactly which chemicals are involved, nor the units in which they
Figure 2: The amount of accumulated dissolved solids in a tank that
receives a 100-percent weekly water change.
In this case the pollution rises each day for six days, reaching a
concentration of between 60 and 70 pu, depending on when exactly the
change is performed on the seventh day, after which the pollution starts
over again at zero, with the cycle repeating week after week.
Those represent two extremes. We can graph any other regimen
in the same way. Let’s start with a common misconception about