~ Frequent Partial Water Changes ~
by Robert M. Fenner

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Frequent Partial Water Changes
by Robert M. Fenner ~ Author of
'The Conscientious Marine Aquarist'
Copyright 2004 www.wetwebmedia.com

Probably the most important aspect of maintenance a keeper of an aquatic system can
do to optimize water quality and health of their charges is to change some of the water
is a regular basis. This Section deals with the reasons for, and some rules of thumb as
to how often, how much and how to make these changes.


There are several major benefits of frequent partial water changes: Dilution of nutrient,
removal of particulate matter, reduction in microbial populations and their metabolites.
Results anticipated are faster, more vigorous growth, reduced algae growth, color and

It has been written in many fisheries, limnological and hobbyist texts that along with
temperature and photoperiod, metabolite ("wastes") build-up is one of the three most
important factors influencing the health, growth and reproduction of livestock.

More specifically; in the trade, ammonia and other nitrogenous wastes are recognized
as the number one killer of aquatic life in captive conditions. Not to say that all the "stuff"
produced by the system's desirable life is toxic. Some metabolites, like pheromones,
are actually known to have calming effects. Therefore the concept of partial, not total
water changes.

In doing these water changes we are interested in a dilution-solution; that is, keeping
these so-called waste products at tolerable levels. There are several ways this is other-
wise accomplished. Most common are some forms of biological filtration and chemical
filtration like carbon and ion-exchange materials. The last are useful but often labor
and money intensive. Moreover, these chemical filtrants do remove desirable chemicals
as well.

As stated in so many previous Sections it is imperative vital that as much extraneous
materials: foods, dirt from decor, material from the immediate outside environment be
kept from getting in the system. What little does make its way in should be removed
by netting/vacuuming, diluted or removed by making partial water changes.

Some potential pollution will probably be added to your system in the way of food and
chemical additives/fertilizer. Even without over- or mis- feeding and/or fertilizing, fresh-
water evaporation adds to a decided negative chemical effect on an aquatic system.
This "Salton Sea Syndrome" occurs as water evaporates leaving behind its' chemical

So enough of the reasoning for making water changes; onto the nuts and bolts of how
to do them:

How often:

Depending on your pump/filter/circulation system, stocking and feeding regimen et al.,
partial water changes about once a week to about once a month are about right. More
frequent smaller amounts are better than infrequent mega-changes, with one possible
exception. Some writers advocate an occasional massive change (50% or more) as a
stop-gap measure to dilute metabolites, nitrates in particular. I'd rather encourage you
to stick to regular, smaller volume changes; they're safer and accomplish about the
same ends.

Make a schedule/notebook for your system and keep track of what you do and how it
works for you. Patterns will emerge and give you a guideline for how frequent you
should change water.

How much:

Five to ten percent for larger systems and twice that for smaller is generally sufficient.
The chemical/physical/biological shock from changing too much too soon is to be

Though some marine authors state that water treatment chemicals are unwarranted
with such frequent small percentage change, I'd encourage you "to be safe, rather than
sorry", and treat to remove chlorine/choramine unless you're preparing water a week
or more in advance of use.

How to:

However it is achieved, the part of the water and what's dissolved in it are mainly to be
found at and in the bottom.

Solids are systematically removed from part, but never all of the bottom of the tank and
possibly sump by using a "gravel vacuum". We don't want to vent all the beneficial mi-
crobes along with the solids, so a plan is made to move the decor and vacuum a half,
third, what have you, of the base in a given water change period.

New water is replaced with pre-mixed synthetic of similar temperature and specific


Regardless of how well a system is designed and constructed, there will always be
maintenance. Frequent partial water changes are one of the best ways of ensuring
continuing success.

There are manufacturers who claim their products do away with the need for frequent
partial water changes. Their products may well extend the amount of time between
changing or ostensibly eliminate it, but at what economic cost?

With the proper tools and materials, water changes are a breeze. Water changing is
the cheapest, easiest, most sure method of diluting wastes and replenishing buffering
capacity, "trace materials", while concurrently cleaning the system of undesirable
solids and liquids.

Bibliography/Further Reading:

Bauman, Edward. 1994. Water wisdom; as if changing a little water will kill you.
AFM 12/94.

Branscome, Lee. 1985. How to stop carrying those buckets of water. FAMA 11/85.

Dow, Steve. 1986. Heavy water. TFH 5/86.

Fenner, Bob. 1999. Frequent partial water changes. FAMA 5/99.

Hanford, Wilber L. 1969. A change of water. TFH 5/69.

Mowka, Edmund J. 1979. Water changes in the marine aquarium; partial water
changes in the marine system are often neglected for a variety of reasons. Here's
why water changes are essential, as well as a method of calculating the necessary
amount. FAMA 12/79.

Ostrow, Marshall E. 1981. Water changes. TFH 5/81.i

Robert Fenner is the Author of the best selling book 'The Conscientious Marine Aquarist'
and 'A Fishwatchers guide to the Saltwater Aquarium Fishes of the World'. He is a marine
scientist and an advid marine aqaurist. Robert Fenner is a former instructor for the Univer-
sity of California system and has regularly contributed to reputable aquarium publications.
Further information regarding Robert Fenner can be found at his website:

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