A common misperception is that sterilisation is utilised in a recirculating aquaculture system (RAS) to prevent disease outbreaks.  The reality is that, although a sterilisation system may assist in combatting the spread of disease organisms, it will not necessarily exclude them.
 
Healthy, happy fish can be exposed to pathogens without contracting the disease as their immune system defends them during attack.  If the fish becomes stressed (poor water quality, inappropriate feed quality or quantity, stocking density too low or too high, rough handling, transport, etc) then the fish’s immune system is weakened and the same pathogen that was previously warded off successfully is now more likely to overwhelm the immunity.  It is thus important to ensure than we farm our fish under close to optimal conditions so that the immunity is robust, and that we exclude pathogens as far as possible.
 
Sterilisation within a RAS is pointless if the source water contains infectious agents that enter the system whenever new water is introduced.  The new water should therefore be sterilised on or prior to entry if there is a chance that it contains pathogens, and this should form part of a wider Biosecurity Protocol.  New fish coming onto the farm must always be subjected to a strict quarantine procedure to limit the risk of pathogens entering the farm via this vector.
 
So the new water entering the system has been sterilised, all new fish have been quarantined and the living environment is optimal for the species of fish being farmed – excellent, but what then is the purpose of the sterilisation within the RAS?  A RAS is a highly fertile, moist environment.  Under these conditions microbes thrive, especially when the fish species is produced in warm water conditions as well.  Two types of organisms may require control, beyond the desire to limit the spread of disease within a system should it somehow bypass the above-mentioned securities, these being bacteria and algae.  If the RAS is held inside a greenhouse tunnel the wet, fertile environment combined with bright lighting can result in the proliferation of planktonic (free living) algae.  These algae compete for oxygen at night and reduce visibility by day, making the system more difficult to manage.  If this is undesirable, you may utilise a weak form of sterilisation to control the algae.  The bacteria that I refer to are not necessarily pathogens they may simply be opportunistic heterotrophs feeding on the organic matter in suspension in the water.  Even so they can be stressful to sensitive fish species, especially the nursery stages, justifying control.
 
Several types of sterilisor can be used in a RAS.  The two most commonly used options are ultraviolet light (UV) and ozone.  UV is installed inside a `pipe’ or `box’ such that the recirculating water flows within 2.5cm of the bulb, as this is the range that UV light effectively penetrates water.  Thus, water exiting the UV steriliser does not contain any residual charge from the UV, so there is no downstream effect or risk associated with its use and it is not possible to use too much UV.  A limitation of UV is that the water must be completely clear and free of solid particles for the UV light to effectively sterilise the water.  Ozone is a radical and highly aggressive form of oxygen and is an effective sterilisation agent as it destroys all organic matter it contacts.  If too much ozone is introduced it will burn up any organic matter it contacts, including the planktonic bacteria and algae, but also the bacteria in the biofilter and body of the fish!  This can be avoided by utilising an ORP meter to monitor the levels of `ozone’ in the water, switch on the ozone generator when the levels are too low and switch it off if the ozone exceeds a set level.  Beyond sterilisation ozone will also remove odours, make the water transparent and enhance flocculation of particles.
 
On a small scale such as a hatchery UV is more cost effective than ozone, but large systems can utilise whichever option they prefer and best suits their design.  Whichever option you choose be sure that the conditions under which the fish are being held is ideal for the species so that their natural immunity will also protect them from possible pathogenic attack.