Well you are really talking about
pods. The plankton you feed is more than likely already dead. Pods will grow in your tank all the time and it is best to have them in high quantities.
Here is how to figure out if you need it on or not. Are you currently fighting a bacteria or problem algae? If your answer is yes, keep it on and just turn it off every once in a while. If the answer is no, turn it off until you have another problem. It is really not good to keep a
UV sterilizer on all the time anyway. It not only kills bad stuff in your tank, but it also kills the good stuff. :?
Another think to think about is... what type of fish do you keep now and what kind do you desire in the future? If you are looking to raise fish that live off
pods, such as seehorses or mandrins, it is important to go ahead and keep that
UV sterilizer off for good. For those types of fish you want a very, very high pod population and every thing you do to the tank now is going to effect when you can put those fish in your tank.