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Nessler is a type of ammonia test kit which shows you the amount of free ammonia (NH3) in the water. It is however affected by the use of water conditioners which convert ammonia into ammonium (NH4) and other similar non-toxic versions.

This type of test kit is usually a dry felt pad of colour (in shades of amber or yellow) and whilst not terribly accurate due to it requiring a judgement based against a rough colour chart within one or two minutes of dipping the pad into the water, they are relatively cheap to buy.