S E P T E M B E R 2 0 0 0
W A T E R Q U A L I T Y
government body ‘The Drinking Water
Inspectorate’. These regulations dictate
the maximum limit of various substances
allowed in tap water. The maximum
amount allowed in tap water for
individual substances is called a
Prescribed Concentration or Value
(PCV). Some waters contain tiny
amounts of a particular substance while
others may contain amounts close to
their PCV limit.
Amounts of substances in drinking
water can vary radically up and down the
country. In one instance, two houses
opposite one another in the same street
were found to have different water. One
house was connected to a plastic main
and the fish health was fine – the other
was served by an old iron-main and the
owner experienced fish health problems.
Figure one demonstrates some of the
variations that can be found in UK tap
water quite clearly.
WHOSE WATER IS IT?
Tap water is designed for people
not for fish
The physiology and the physiological
needs of people and fish are different.
When was the last time one of your koi
came into the kitchen and poured itself a
glass of water?! All good fish keepers’ first
priority should be to change their human
water into fish water.
Humans are at the top of the animal
hierarchy and can therefore tolerate
larger amounts of substances than fish.
Figure two demonstrates the difference
between safe levels of substances for
long-term ‘good’ fish health and the
PCV limit for tap water under the
In some instances humans require
‘extra clean’ water, for example for
medical purposes such as renal dialysis.
Figure three, on page 24, illustrates that
even water used for this purpose still
contains amounts of some substances
higher than those tolerated by fish. Quite
a sobering thought! You could assume
that if substance values in our water were
no higher than the ‘fish safe’ values
shown in figure two then everything
would be fine for our fish, but
unfortunately it’s not that simple.
Metals and pesticides ‘fish safe’ levels can
be affected by the way that they inter-
react with members of the same group,
for example metal with metal and
pesticide with pesticide. This inter-
reaction is known as a synergistic effect. FIGURE
PCV (1989 UK WATER REGULATIONS)
OF POTENTIAL VARIATIONS Total
and Free Chlorine No Specific Standard
00.00 mg/l – >1.00 mg/l Pesticides,
<0.01 µg/l – 0.1 µg/l
3 pesticides – over
80 individual pesticides.
0.5 µg/l, total
<0.01 µg/l - 0.5 µg/l
The values in this box only apply if a water
company has ‘softened’ the water.
Minimum 60 mg Ca/l
c. 6 mg/l - over 300 mg/l
Minimum 30 mg HCO3/l c.
6 mg/l - over 300 mg/l Aluminium
<5 µg/l - 200 µg/l
µg/l <5 µg/l - c. 1,600 µg/l
µg/l - c. 200 µg/l Lead
50 µg/l <2
µg/l - 50+ µg/l (for houses with old lead
50 µg/l <5
µg/l - 50 µg/l Zinc
5,000 µg/l <
5 µg/l - a few hundred µg/l Note: Under
the latest regulations the lead PCV is being gradually reduced
= parts per million µg/1 = parts per billion <
= less than > = more than FIGURE
FISH ‘SAFE’ VALUE
TAP WATER PCV
mg/l No Specific Standard See Figure 1.
koi: minimum 100 mg/l Untreated,
No PCV (Seen as low as 6 mg/l) Aluminium
50 µg/l 200
µg/l Copper, soft
water 5 µg/l
Copper, hard water
15 µg/l 3,000
30 µg/l 200
20 µg/l 50
µg/l (See note Figure 1) Manganese
10 µg/l 50
30 µg/l 5,000