I saw some devices like this being sold for sustaining copepod populations in an aquarium and thought I could make that myself and maybe even improve on the design.

The design features 5 3.5-inch plates with a corrugated design that is easy to print without supports and gives plenty of surface area.  These are contained by a housing and lid.  

The plates have various holes and passageways that allow for movement between all the different areas without leaving the unit.  

The lid features a handle to make it easier to pick up the unit and it also has 4 holes on the top that you can use to inject more copepods or food for the copepods.

The housing features a sump area that allows it to drain while retaining some of the water in the bottom.  The thought was that many of the pods would remain in this sump area when the unit is lifted out of the water.

I can't see the copepods or at least differentiate them from various other aquarium specs, so it's hard for me to say definitively if the copepod housing units are working.  However, it has been a couple of months and my mandarin fish hasn't died.  I have two units in my sump and every week I take one of them out and swish it in the display tank.

I made the units out of PETG as I believed this to be more aquarium-safe.  

Update: I've also added a 3.5+ version of the housing.  This features a thicker base and arms which add more rigidity and the possibility for a weightier assembly.  Just be sure to print with sufficient infill or wall thickness to ensure that it sinks.

I printed mine at 100% infill.

You can also check out my other handy aquarium related models.

Glass Canopy Aquarium Chute:

https://www.printables.com/model/760099-aquarium-chute-for-glass-canopy

Bucket Clip:

https://www.printables.com/model/359202-bucket-tubing-clip

I designed this using Solidworks.