After I bought a Fluval Flex 35l/9g (okay... just because it looked nice) I quickly noticed some flaws and / or opportunities to tinker :-)

So I started to redesign and add some things. This is Part 2 (Part 1 is a fan mount for cooling the water https://www.thingiverse.com/thing:3640543) of the project.

The whole project will cover cooling, heating, light control, water refill (and whatever idea I will have in addition) using an ESP micro controller and without any destructive modifications at the tank hardware.

The original Filter has a really strange arrangement of the filter media. Generally you want the water to flow first through mechanical filtering, then through biological and then sometimes through chemical (like active carbon treatment or nitrate removal).
In the original setup the water was more or less routed through the mechanical filter and maybe or not it could touch the biological or (less likely) the chemical one.

So I designed an insert instead of the original sponge block which can take 3 separate baskets and ensures a flow direction top -> bottom.

At my current setup, middle basket is filled with EHEIM substrate pro, the upper one with a piece I cut out of the original sponge. The space above the upper basket I loosely stuffed with filter wool.

I added also some half height baskets to combine for example filter mats or fleece.

For me it now works perfectly fine and the EHEIM substrate outperforms the original fluval bio filter by... a lot. Nitrite is always below 0.2mg/l (ppm).

The models should be printed in the orientation of the STLs (maybe rotate the case 90° around Z).
All models can be printed without support
The baskets can be printed without support, assuming your filament profiles (bridging) are dialed in well. The bridge length is 20mm. Otherwise please use supports.

Print:
please check the dimensions of your filter compartment first. There is a _127 version of the Case which is about 2.5mm shorter (107.5mm) than the _57. The Handle_57 and Basket_57 are the same for Case_57 and Case_127

please use the files for your tank:
_57 for the 127L/32.5g and 57L/13g
_35 for the 35L/9g

1x Case
1x Handle
3x Basket (or 2x Basket_half for 1 Basket)

Pay attention to insert the baskets with the small holes facing towards the open side of the case

The Case is inserted into the tank with the intake holes to the left (standing in front of the tank) - not the side where the pump is.

Use PETG, PA or any other food safe material and water the model for at least 24h before inserting it into the tank. If you plan to switch to other bio filter media, insert it in the lower basket and the used one in the one above. leave this for at least 4 weeks before you remove the old one.

Huge thank you to:
Trigg-Darren for providing the dimensions of the 57L and for testing!
jtreminio for the 107.5mm Case_127 idea

[UPDATE 13/JUN/2019]

• added half height baskets
• small correction on basket_57

[UPDATE 29/AUG/2020]

• added 107.5mm version of the case to match some 127L / 32.5g Tanks

published project parts so far:

• Fluval Flex Cooling Fan Adapter 40mm
  https://www.thingiverse.com/thing:3640543
• Fluval Flex Cooling Fan Adapter 80mm
  https://www.thingiverse.com/thing:3671625
• Fluval Flex Filter Mod
  https://www.thingiverse.com/thing:3639353
• Suction cup temperature sensor mount for DS18x20
  https://www.thingiverse.com/thing:3640844
• water level controller hose clip for Fluval Flex filter compartment
  https://www.thingiverse.com/thing:3675744
• Fluval Flex Intake Pre-Filter / Baby-Safe
  https://www.thingiverse.com/thing:3730658