In the wild fish live in a wide spectrum of water types, representing clear, murky and completely opaque water.  As fish farmers we can also find that our water can range from clear to extremely murky, but which is best for farming fish and how do we maintain these conditions?
 
Types of turbidity
Different types of turbidity impact the transparency of the water on fish farms, and this is often related to infrastructure type.  Earth ponds have the greatest variation in terms of water clarity and can be crystal clear, green due to planktonic algae, brown caused either by tannins or decomposing grasses, or totally opaque as a result of clay suspended in the water.  Water in recirculating systems is generally clear but can carry high loads of fine suspended solids, be green from algae or even be milky due to bacterial activity.
 
Effect of the turbidity
Crystal clear water is wonderful for the operator as you can clearly see the systems and fish in order to manage the situation effectively.  However, fish can easily stress in clear water, especially shallow clear water with bright light, and most especially if the fish are held at a low density.  So, some opaqueness is desirable for the fish, but if the water is too murky it is no longer possible to assess the behaviour of the fish.  In addition to this, algae and decomposing grasses can each result in fluctuating pH and low oxygen dissolved in the water.  Suspended clay is undesirable from every perspective!
 
Managing water clarity
The management of the algae density in earth ponds is a weekly function whereby we assess the clarity and only fertilise to maintain a clarity of around 30-50cm to ensure there is abundant natural feed for the fish.  If there is suspended clay in the water this can be settled out cheaply by throwing dry grass into the pond, allowing the clay to settle, and then using the water elsewhere.  If the pH fluctuates by more than 1pH point per day, stabilise it by buffering with calcium carbonate.  Low dissolved oxygen can be rectified through aeration.
 
In a recirculating system the suspended solids should be removed by means of mechanical filtration, whereas the algae and bacterial activity can be eliminated through the use of UV sterilisation.
 
The clarity of the water on a fish farm seldom receives direct attention which is unfortunate as it is an important factor in the greater management of fish stress and therefore fish health.
 
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