Water testing

...when, how and why

Water testing: Fish health & koi health

It is almost impossible to overstate the importance of water quality to fish health. Equally it is impossible to assess water quality by simply looking. The only way that we can be assured that conditions are right for fish is to regularly test for the important water parameters.


Test every week - more frequently if the system has been disturbed or medicated. The time of day when ponds are tested is important because of the effects of photosynthesis


Test every week - more frequently if the system has been disturbed or medicated

nitrite (NO2-)

Test every week - more frequently if the system has been disturbed or medicated

nitrate (NO3-)

Once monthly in freshwater. every week with marines, corals and invertebrates

general and alkaline hardness

Once monthly


Weekly in marine tanks

ORP (redox)

Optional in fresh and marine - of more use in marine tanks




Necessary when copper treatments are used


Optional - but used in conjunction with nitrate testing it gives an measurement of background pollution. fresh water & marine

organic pollution


dissolved oxygen

Use when water temperatures increase. It is important to try and take a sample from within or under the filter media if possible, as filter levels may be lower than in the pond.

Additional tests 

Other mineral tests are available for marine. i.e. calcium etc. Go to specialist dealer for information

It is important to keep a record of water test results, as this will highlight seasonal fluctuations and help pin-point water quality problems in the event of disease outbreaks.

Water testing equipment is available in test kit form using either liquid reagents or tablets. In general these are accurate enough for hobbyists uses. Electronic equipment is more accurate, but somewhat more expensive.

Mardel Clout is safe in tanks and ponds, minimal to no effect on plants and bacteria, safe for scaleless and sensitive fish. Dosing in this website.

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Expanded Content by Dr. Erik Johnson, DrJohnson.com and Used with Permission; Frank Prince-Iles ©2009 All Rights Reserved