Air valves must be fitted in the two closing lids of the mini-biotopes, which allow inhalation via the biotopes but prevent exhalation. Exhalation takes place via corresponding valves integrated into the diving mask. I use a valve that can be found in some FFP2 masks for this purpose.

I drew stencils in Inkscape and applied them to the lids using spray adhesive. Then the cutouts were sawed out with the scroll saw, sanded, and cleaned with isopropanol before the air valves were glued in place with 2-component epoxy glue.