wait can i put the UV light inside the tank? Because i dont have a sump. I have two fluuval fx6. i heard that putting a UV light inside the tank will kill the fish as well. I mean now my DT is fish less because they are all in a QT. Also what brand UV for a 100 gallon.My experiences with and without UV have made me a believer in UV.
My recommendation...
Add an appropriately sized UV sterilizer to your DT and it will help tremendously in reducing the spread and proliferation of parasites. Personally I recommend 18-watts / 125 gph flow rate with a targeted tank turnover rate of 2-3x/hour.
Using a 100 gallon tank as example; that means targeting a flow rate of 200-300 gph through a 36-watt UV sterilizer. You can accomplish that with a separate closed loop pumping system or as part of your return.
Using UV, you may not prevent a single fish from succumbing to parasites, but you can/will slow the propagation of the truly aggressive ones like Velvet, enough to give you time to segregate, diagnose and treat accordingly before it spreads around your tank.