Testing water. How and why?

Keeping fish is not like keeping any other pet, such as a cat, dog, rabbit or guinea pig

as they are unable to escape from their pond. If four-legged pets take a disliking

to their surroundings or become stressed in their environment in which their owner wishes

to keep them, they can retreat for cover until the hazard has passed.

This is not true for fish, which, whether they like it or not, are stuck with the

environment we provide for them, whether it is stressful to them or not. How we manage

and maintain the water in a pond/aquarium will have a direct effect on the health and

growth of our fish. For example, fish are constantly excreting toxic waste into

their own living space and they will eat, breathe, and even drink that same water. We

must ensure that we can optimise the water quality experienced by our pond fish, otherwise

we should expect a string of health and disease problems.

How can I optimise the water quality in my pond/aquarium?

Fish in natural lakes, ponds, rivers and oceans fair very well where stocking rates are

typically a lot lower than in a garden pond and where a balanced, thriving ecosystem is

maintaining a healthy water quality. Problems can occur in a small pond/aquarium

where the number of fish can be unnaturally high. This is accepted as the norm,

and should not lead to a decline in water quality if a number of guidelines are followed:

A pond/aquarium filter is an essential requirement. It’s job is to remove solid waste

from the pond, keeping the water clear and to breakdown the soluble and invisible toxic fish

waste in less toxic by-products. This is achieved over time by beneficial bacteria that

naturally colonise the filter media.

Feed fish sparingly, bearing in mind that the amount of food eaten by fish is directly related

to the amount of toxic waste they excrete. If too much food and too many fish are added

to a pond/aquarium then water quality and fish health problems are likely to arise.

Carry out regular partial water changes. About 10-20% of water should be changed every

3-4 weeks to dilute the build up of stubborn toxic by-products. A water change will also

‘freshen-up’ a pond/aquarium, having a noticeable and positive affect on fish behaviour.

Endeavour to treat raw tap water with a conditioner when carrying out a water change as tap water

may be good enough to drink, but is not guaranteed to be fish friendly.

Add aeration. This is easily achieved by placing the return of the filter down a waterfall

or attaching a fountain to a pump. The extra agitation of the water will increase to

dissolved oxygen content, which can drop worryingly low in the warmer summer periods.

What affect does water quality have on fish?

Water quality has a very direct and predictable influence on fish health.

Poor water quality = Poor health

Good water quality = Good health

As fish are unable to escape poor water quality, they must suffer the stress of swimming in a

soup of toxic compounds which they have excreted – not a healthy situation. This

makes fish more susceptible to disease, and a downturn in water quality is usually causes a

change in fish behaviour.

Watch out for:

Loss of appetite

Sulking on the pond bottom

Hanging motionless at the surface

Clamped fins

Gasping at the surface

All these changes in behaviour will usually indicate a deterioration in water quality.

What is good water quality?

Good water quality is a measure of how suitable water is for keeping fish healthy.

There are a number of simple tests that can be carried out on pond water to determine whether it is suitable.

The first thing to do upon noticing a change in your fish’s behaviour is to carry out a water test, to determine

the cause of the problem, which will allow you to resolve the problem, preventing it from happening again.

What do I test my water for, and what am I looking for?

Recommended Water Quality Criteria are set out in more detail by OATA

- See http://www.ornamentalfish.org/coc/quality/quality.htm

There are well over a dozen different test kits available, each testing for a specific water parameter.

Do not let this put you off, as there are only really a handful that you should need to use to give you

useful information rapidly.

pH. This is a measure of how acid or alkaline the pond water is.

Pond water must be above 7 and no higher than 8.5, or 9 at a push.

If upon testing your water is within this range, then leave well alone. Tap water is made alkaline by

water companies, so regular water changes should help to keep your pH acceptable.

If your pH is below 7 (acidic), then a water change or the introduction of lime stone chippings in the

filter will bring it back up to the desirable range. If your pH is above 9, and too alkaline, the

probable cause is the running in of untreated lime into the pond. Seal any surrounding cement work

with a clear plastic sealant.

Ammonia. This tests whether the toxic waste (Ammonia) that fish excrete is being broken down.

An ammonia test is only usually required when a new pond and filter are being matured over the

first couple of months and there is a question over the functioning of the beneficial bacteria in the

filter. The desirable ammonia reading is zero, but should a positive reading occur then carry out the following:

1. Stop feeding

Do not introduce any new fish

Carry out a 20-30% water change

Only start feeding when reading is back to zero (this may take a few days).

Carry out a test each day for the next week.

However, if a positive Ammonia reading reappears upon daily testing, carry out steps 1-4 again.

As the filter matures, an ammonia reading is less likely to occur.


Nitrite is a persistent toxic by-product of the bacterial breakdown of ammonia. Although it is slightly

less toxic than ammonia, the bacteria involved in breaking it down further are slower to act than those

involved in the breakdown of ammonia.

The desirable nitrite reading is zero, and if a nitrite reading is present, it is an indication that the

filter is not coping with the amount of waste being produced. Even a low nitrite reading is undesirable

and if present, the same procedure (1-4) for ammonia toxicity applies. Similarly, the water need only be

tested for nitrite in the early weeks of a pond and filter, while the bacteria responsible for its breakdown

become established. Once the filter bacteria have colonised to the level of your stocking and feeding rate,

a nitrite reading should not occur again as long as the feeding and stocking rates remain pretty constant


As the pond or aquarium matures and the biological cycle of filtration progresses, the nitrate levels are

likely to increase. Nitrates can be thought of as a ‘bank’ where nitrogen that is excreted by fish as ammonia

is broken down into nitrite and eventually deposited as nitrate. Nitrate can be reduced by plant growth or

anaerobic bacterial action, but these processes are usually limited by other factors, reducing the likelihood

of their natural removal. The most reliable method of removal is a partial water change with low nitrate

water. The solution to pollution is dilution.


KH is a measure of the dissolved bicarbonate ions in water. Bicarbonates have a high buffering capacity,

directly influencing the pH of water and its overall stability.

A high KH shows a well buffered water which is likely to have a high pH that is stable and will resist movement.

A low KH shows that the water is unbuffered, is likely to be neutral or slightly acidic and will have a

tendency to become more acidic rapidly in an aquarium.

Amazonian – Low KH so that a neutral/acidic pH can be maintained

Community – Moderate KH so that a slightly alkaline pH can be kept stable.

Rift Valley (and Marines) – High KH so that pH is high with no tendency to drop.


GH is a measure of water hardness caused by magnesium and calcium ions.

Species of fish have specific water hardness requirements and there is a link between GH and pH although

it is not as close as KH. I.e. A high GH is less likely to influence pH than a high KH.

How do I test?

Test kits typically use a change in colour to measure a water parameter. A sample of water is taken from the pond,

filling a clear test phial. Either a series of liquid re-agents or a tablet are added (depending on the manufacturer),

and the sample is left for a set time, waiting for the colour to develop. The final colour is simply compared to a

chart to determine the reading. It’s as simple as that.

Nothing substitutes for experience

Once you have kept fish healthily for a season or two, and managed the water quality in a pond successfully, the need to

test your pond water regularly should reduce. It should become possible for the experienced eye to assess the quality

of your water by simply assessing the behaviour of the fish. You’ll be surprised as to how accurate your observations

can be and how your experienced eye can become as accurate as the best test kit. It can certainly work out quicker and cheaper!

Kill blanketweed and string algae.