Fluval Flex Lower Inlet Cover

The Problem with the Stock Fluval Flex AIO

The stock 9-gallon Fluval Flex AIO design is poorly optimized for saltwater use, especially concerning the rear filtration section. The unorthodox twin-intake design results in the AIO water level matching the display water level. This causes minimal actual surface skimming, which is needed for a healthy reef tank. This high water level offers a very small margin of error if your Auto Top Off (ATO) system were to fail.

My Optimization Journey

To optimize the Flex for saltwater, I made a lot of modifications:

Lid Replacement: I removed the stock lid. Excellent replacements are available, including this 3D-printed option

Media Basket: I am using the media basket designed by uepsie which can be found here: Fluval Flex Aquarium Filter Mod

Light:  I bought a cheap Hygger 36 Watt light 

Return Pump Upgrade: I swapped the stock pump for a more reliable option, currently using the SICCE Syncra Silent 0.5. I also upgraded the tubing from clear vinyl to inexpensive black pond tubing (available at most hardware stores).

Flow Enhancement: Investing in a quality wave maker (brands like Hygger, Jebao, or Sicce offer good fits) are required for proper water circulation.

P.S. It's frustrating how many useful modifications online are locked behind paywalls.