AN
ADVANCED AQUARIST SHORT TAKE by D. WADE LEHMANN
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Necessary
Maintenance - Water Changes
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There
are few things in this hobby which are static or even thoroughly
understood. The effects of regular water changes are no exception.
I will illustrate for you exactly why I believe that water changes
are an integral part of the regular maintenance on reef tanks
and how much should be changed at once.
I constantly
hear people say that they have never done a water change on
their tanks. I have heard people say just the opposite; that
they do very regular water changes. I have seen tanks that look
great with both and tanks that look horrible with both - water
changes are not a cure-all for everything nor anything specific.
That being said, I will say that in my years of reefkeeping
experience, almost all tanks that have regular schedules of
water changes look the best.
Water
changes are a relatively low cost and easily attainable portion
of a regular maintenance regime. They help by removing unwanted
waste products and possible contaminants. They are also beneficial
in the replacement of those nutrients and elements, which are
normally consumed in the tank and not readily replaced. I have
even seen tanks that use water changes as a sole source of replenishment
(no additional supplementation). While that is not feasible
for most of us due to time, cost, and headache, it would be
ideal.
One
key item to be aware of is the dilution effect of water changes.
In order for a water change to be useful, you must remove a
portion of the water entirely and then replace that portion.
If you add new water to the system, then remove the same volume,
you are actually reducing the value of your new water significantly
due to dilution. In effect, you are removing some of the new
water you have just added, although in reality you dilute down
the compounds you are trying to remove and then removing a portion
of that diluted water (figure 1).
Figure
1 - Diagramatic representation of two forms of water
changes. (Top) Removal followed by new water replacement.
(Bottom) Addition of new water followed by removal of
equal volume. N represents a contaminant concentration.
Automation
There
are different methods for replacement of aquarium water with
varying degrees of ease. Most people drain their sumps and replace
the old tank water with new water using a siphon or scoop and
a few buckets. Some have elaborate systems of valves and plumbing
as an integral part of their system that allow simultaneous
removal and replenishment from two differing areas of the setup.
I perform
mostly regular bi-weekly water changes with a 30g mixing trash
can, a Mag 7 pump, and some tubing. I run new water into the
sump while simultaneously diverting the main return line into
a second 30g waste container without stopping the return pump.
This allows for maximal ease and due to the large volume of
my sump, little to no mixing of new and old water prior to disposal.
Automation,
in general, allows for an easier more hassle free water exchange,
which as we all know makes it an easier and more readily completed
chore.
How
much should I change?
The
portion changed, or percent of system volume replaced, is a
critical issue when talking about removal of possible toxicants
and toxins, and replacement of valuable elements and nutrients.
Consider
this hypothetical situation. You are cleaning up around the
tank, when someone walks by and accidentally spills a cleaning
solution into your tank. What do you do? Once the initial panic
subsides, you realize that you need to perform some water changes.
The chemical, luckily, is not very toxic, but it is still a
concern and using activated carbon and a skimmer will not remove
it. So, you have a bucket of prepared saltwater… is it
enough?
Figure
3 illustrates the remaining fraction (in %) of the compound
after water changes of varying sizes. Ten, twenty, and thirty
percent changes have differing effectiveness in removal of the
cleaner.
Figure
3 - 10, 20, 30, and 50% water changes and their dilutory
effects. Note the effectiveness of larger % changes.
The half-life of a hypothetical compound in the aquarium
(t1/2 or time in changes to remove half) occurs at 8,
3, 2, 1 changes respectively.
If
you were to perform 10% changes on your tank to remove the cleaner,
it would take you 23 changes of water to get the level of the
compound in the tank down to less than 10% (9.85%) of the starting
concentration. Likewise at 20% and 30% volume changed, it would
take you 11 and 7 water changes to bring the level down to 8.6%
and 8.2% of the starting concentration respectively. While I
would not recommend a 50% water volume change for most tanks,
it is very much more effective in eliminating the original spill
of cleaner. It would take only 4 water changes to bring the
level down to below 10% (6%) of the original concentration.
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The
risk, at 50% water volume changed, typically comes from shock
and stress to the occupants of the tank due to salinity, temperature,
and other physical-chemical differences that occur between old
tank water and new water. For this reason, unless it is a true
emergency situation, I do not promote changing 50% of the water
volume.
The
case of the 10% water change is also negative. While 10% is
better than no actual volume changed regularly, it will take
a great deal of time to remove any potential threats and replace
those compounds that are valuable to the tank. In figure 4,
it illustrates 10% changes and the effect of such a small change.
Thirty consecutive water changes of 10% total volume means that
you will still be left with almost 5% of the starting concentration
of contaminants. In line with that consideration is that fact
that at best, 30 water changes will take half a year to complete
at more than one per week.
Figure
4 - Illustration of the % remaining and the actual parts
per million (mg/liter) remaining assuming 2ppm addition
of a toxicant.
The
effects of water changes are exemplified in cases where people
have underpowered skimmers, no skimmer at all, or very small
tanks. Changing water on a regular basis can have the same effect
as running a skimmer at removal of organic wastes, although
the lack of a skimmer will allow for more bacterial decomposition
of the compounds (less real-time removal) prior to elimination.
After
considering the result of changing varying percentages of water
volume, I highly suggest 20 or 30% water changes, performed
on a semi-regular basis. Usually every 2 to 3 weeks or whenever
conditions in the tank appear to change or the water turns somewhat
yellow due to the organic acids from decomposition building
up.
Water
change volume table
Water
Change Volumes (in gallons) Per Tank Size
Tank
Size in gallons
Estimated
Actual Volume
Estimated
Water for 20%
Estimated
Water for 30%
10
7
1.4
2.1
20
14
2.8
4.2
29
20.3
4.1
6.1
40
28
5.6
8.4
55
38.5
7.7
11.6
75
52.5
10.5
15.8
90
63
12.6
18.9
120
84
16.8
25.2
125
87.5
17.5
26.3
150
105
21
31.5
185
129.5
25.9
38.9
300
210
42
63
The
table gives the most common size tanks and their nominal and
estimated volumes in gallons. The estimated volume is 70% of
the maximal tank volume as water assuming that sand, rock, and
various forms of life occupy the remaining space. Twenty and
thirty percent change volumes (in gallons) are shown.
