Yesterday I put the link to the Thingiverse project to be able to download the STLs but since then there have been improvements on the small tank. 

Before it was in 2 parts that had to be screwed together which could lead to a lack of watertightness. For the tests this was not very important as the water falls back into the bottom tank. Due to energy consumption it is better that the water pumped in the small tank stays in it.

The drain holes have been enlarged and the hole for the pump hole moved  up to avoid suction when the pump is stopped.

In addition, the one-piece print is a time-saver.

First Version:

Second version:

The new STL can be found in the files of this project under the name: small_tank_V2

All the 3D printed parts are made with recycled PET bottle filament (more information in this project of my father)