If you are sticking with the with the stock pump for that tank you actually have a few options. The stock pump is lsited as a 1000 L//H (264 GPH) pump. that's enough to overdrive a singe 1/4in RFG or supply just the right amount for a dual 1/4in RFG setup.If I have a 25 gallon peninsula with the stock pump what size would I need? Probably the 1/4?
You could also do a single 1//2in , but it would be at the lower end of the suggested flow rating for hat RFG,
Here are your two best options
Dual 1/4in RFG
You'll need the Ultimate Nano Tank Return Line Upgrade kit and a 16.5mmWaterbox adapter. You'll find both of these items listed at the bottom of this page.
Ultimate Nano Tank Return Line Upgrade Kit – Dual 1/4in Flex-Series RFG | Vivid Creative Aquatics
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Single 1/4in RFG
the Waterbox® Cube Flow Kit with 1/4in Random Flow Generator is a pre-configured Flow Kit - so it comes with everything you need.
Waterbox® Cube Flow Kit with 1/4in Random Flow Generator® Nozzle – WBFK025X1 | Vivid Creative Aquatics
vividcreativeaquatics.com