Apart from the relatively simple operation of
trapping metals particles, we now have to demand that
purifiers also reduce dissolved metals. Before you can
do that we are back to we ‘need to know’ the total
amount of metals in the water and whether the water
is soft or hard to determine the amount of work a
purifier has to do i.e. how long a certain amount of
purification medium will be effective at protecting your
koi and how often you will have to change the
If a purifier is not capable of reducing metals,
particularly the dissolved form, metals will still end up
in the pond even though the water has been through a
purifier. If you don’t know when to change the
purification medium in a well functioning purifier for
your tap water, then you seriously risk exhausting the
media and allowing the now untrappable metals to
march straight through the purifier and into the pond.
If you do have an appropriate purifier and have
worked out purification medium exhaustion for your
own tap water, but fail to change the purification
medium soon enough, the metals will again march,
unchecked, through to the pond. Remember ‘time’ for
change of media is always measured in total metals
(etc.) and gallonage, never in weeks or months.
You cannot rely on the blocking of a purifier as a
sign that the filtration media is now exhausted.
Certainly we have test results to hand which show
where both granular and block media have become
exhausted even though no signs of blocking have
You might be doing everything right but, towards
the forecasted end of the effective life of the media, the
fish unusually flick and flash when adding new water.
This will mean that the incoming water has changed,
higher than anticipated levels of metals (or another
substance) have entered the purifier, and the
purification medium is now exhausted or ineffective.
Your koi are your in-house experts on water quality.
Their flicking and flashing is telling you to throw away
the rule book and change the purification medium or
media immediately! If they continue to show
discomfort, seek help.
Some forms of purification media available to the
fish keeper are just totally unsuitable for koi protection,
sometimes because they just don’t do enough and
sometimes because of what they actually do.
A koi keeper gave me a purification cartridge at a
koi show which he had bought for reducing a range of
metals for his koi, the cartridge measured about 10in in
height and 2.5in in diameter. *The American cartridge
was manufactured to reduce free chlorine, entrap cysts
and contained a special medium to reduce lead. (The
only specific ‘metal’ reducing medium I have traced,
reduces just lead.) It is very rare to meet a lead problem
for koi in the UK, the only instances I now see tend to
be directly linked to old houses with the original lead
Nevertheless, the koi keeper had thought the
cartridge would reduce all dissolved metals - not so -
aluminium, copper, iron, manganese and zinc would
march straight through the cartridge unhindered.
Where water needed minor ‘tweaking’ for fish the
cartridge may have been acceptable (dependent upon
how and where it was used) for the reduction of free
chlorine and perhaps some pesticides. But it did not
offer the fish protection the UK koi keeper sought, i.e.
the reduction of mixed, dissolved metals.
Some koi keepers seem to expect that purification
media will treat more water than the main-line
purification (filtration) world know they will. The same
koi keeper thought this cartridge would not only reduce
all metals but that it would do so for many thousands
of gallons of water. *The American manufacturer’s
rating is for 2,000 U.S gallons i.e. only c.1,666 Imperial
Traditionally for domestic purification many
‘metals’ cartridges are given an effective lifetime rating
based on the reduction of lead, even when the
cartridges are capable of reducing a variety of metals.
This is for two reasons: (1) Lead used to be a wider
problem in drinking water than today. (2) Rating on
lead maximises the life of a cartridge and could be seen
as a good sales gimmick as the cartridge then appears
to have a wonderfully long life.
Let’s explore that a little further. Lead is the easiest
of all the dissolved metals to reduce and uses the
smallest amount of purification media. As soon as there
are more metals than just lead in the water (and there
always are) the effective life of the cartridge is suddenly
Take a new resin cartridge (the same size as before
ie. about 10in x 2.5in) for each of the following tests‡:
Base all tests on the use of dissolved metals. Maintain
the same flowrate in all tests. Influent
Metal Amount Effective
Cartridge Life Lead
50 ppb c.18,400
Imperial Gallons Aluminium
50 ppb c.
9,300 Imperial Gallons Aluminium
200 ppb c.
2,325 Imperial Gallons For
the treatment of water for koi we have ended-up
with three totally different effective lives for one
identical cartridge. Forecasts were based on two
different types, as well as two different amounts, of
metals. Why those particular tests? K O I
13 * Based
on a manufacturer’s data sheet. ‡
Based on testing undertaken at a NAMAS accredited laboratory.