If u have a 8 watt and you are running 500 gallons per hour, i believe it really isnt doing anything. The flow rate is supposed to be slow to kill bacteria and even slower to kill parasites. The 9 watt that im gonna get says suggested flow at 100-200 gallons per hour and the lower the wattage the slower the flow rate. So you might want to check that out.
*Im not 100% positive but Im pretty sure this is true.