A Double-Helix 9-watt is rated at 200 gph max flow rate. The AquaClear 402 is rated 270 gph max flow. If you are running your 402 at full flow, then you're over-flowing your sterilizer. I would suggest either turning your 402 all the way down and see if it helps, or going with a 301. An ideal powerhead for the Double-Helix 9-watt is the ViaAqua 480. It also hooks up easily to the Double-Helix. This is the setup I have provided for customers before, when they requested a sterilizer. HTH