S  E  P  T  E  M  B  E  R     2  0  0  0 2 3 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
1989 regulations.
     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.  
SYNERGISTIC EFFECTS 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 ONE SUBSTANCE PCV (1989 UK WATER REGULATIONS) EXAMPLES OF POTENTIAL VARIATIONS Total and Free Chlorine No Specific Standard 00.00 mg/l – >1.00 mg/l Pesticides, individual 0.1 µg/l Quantity
<0.01 µg/l – 0.1 µg/l
3 pesticides – over
80 individual pesticides.
Pesticides, total 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.
Total Hardness Minimum 60 mg Ca/l c. 6 mg/l - over 300 mg/l Total Alkalinity Minimum 30 mg HCO3/l c. 6 mg/l - over 300 mg/l Aluminium 200 µg/l <5 µg/l - 200 µg/l Copper 3,000 µg/l <5 µg/l - c. 1,600 µg/l Iron 200 µg/l <5 µg/l - c. 200 µg/l Lead 50 µg/l <2 µg/l - 50+ µg/l (for houses with old lead plumbing) Manganese 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 Table: mg/1 = parts per million    µg/1 = parts per billion    < = less than    > = more than FIGURE TWO SUBSTANCE FISH ‘SAFE’ VALUE TAP WATER PCV Chlorine 0.03 mg/l No Specific Standard See Figure 1. Alkalinity For 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 3,000 µg/l Copper, hard water 15 µg/l 3,000 µg/l Iron 30 µg/l 200 µg/l Lead 20 µg/l 50 µg/l (See note Figure 1) Manganese 10 µg/l 50 µg/l Zinc 30 µg/l 5,000 µg/l