I have a 55w UV unit and my aquarium is a 150g dt with a 50g sump.Depends on what you are trying to treat. I have a Pentair Smart 25 watt on my 120 that I run at 80 gph for parasites as my tank is fishy heavy. For algae/ bacteria it recommends 450+ gph. The Apex flow meter is nice to have for this.