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"You might question the value of the UV sterilizer. It kills the 
nitrogen cycle bacteria. Every healthy aquarium I've seen has some 
valuable bacteria cultures. Although you have a low reading of 
Nitrite, this is an indication that there isn't nitrifying bacteria 
that will transform Nitrite to Nitrate."

I wouldn't blame the UV for a lack of nitrifying or denitrifying
bacteria.  Maybe if you started out with a sterile tank and ran the
entire tank full of water through the UV and then into a holding tank
before you sent it back to the original tank, then the UV might prevent
the cultures.  But typically the return water from the UV goes into the
tank and mixes with water that has bacteria.  Although you keep drawing
water out, it mixes with the tank water when it returns.  If there was
perfect mixing, it would take several hours , 8, 10, 12, to pass UV
over every water molecule (see for example the book, Aquatic Systems
Engineering, by P.R. Escobal).  If there was no mixing (for example
using a holding tank), it could happen at whatever the effective rate
is for your pump.  But neither case is realistic.  A UV can hold down
water borne bacteria levels but is not so effective at sterilizing
water that remixes with non-sterile water.  And the propagation of the
nitrifying and denitrifying bacteriacultures is on surfaces in
biofilms, not waterborne  -- although waterborn is one effective method
of contagion.  That's why starting with a sterile tank, and perfect
sterilizing of all of the water without mixing sterile and nonsterile
water could prevent some biocultures from starting.  But it woul dbe
difficult, in pratice, to set up a situation where that would happen,
even with a UV.

Scott H.

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