It's been a long time since I've used any polyquat so I had to look up the dosing information. What does it say on the label? I believe, unless it has changed, that the dosing is 9 oz. per 10,000 gallons of water. Please look at the label and see if that is correct and let me know. And yes, you can add it in front of the return. You also might want to put your filter on recirculate for awhile so it doesn't filter out so fast.