UV filter

need some help with installing a UV filter please. I have a rural well that free flows into a 200 gallon cistern in the basement. from there it is pressurized , run thru 3 filters, water softener then on to the house. I'd like to install a UV filter to kill any little live organisms, etc that may be in the water. the water flows into the cistern at about 50 gallons/ hour. my question is: can I install a 2 gallon/minute UV filter on the supply line going into the tank (thereby killing everything before it enters the system) or do I have to get a larger (ie. more expensive) UV filtr and install it after thetank where the flow rate is maybe 5-6 gallons/minute? any input would be appreciated