Passive CO2 diffuser bell for use with low-pressure DIY CO2 generators.  I have used these for years successfully when you just need a bit more co2 than the atmosphere provides.  

Printed with PETG, .3mm layers, no supports necessary.  Once printed it uses two suction cups to hold it to the side of the aquarium.  I use standard 3/16" aquarium line attached to the co2 generator then inserted into the top hole.  The stand pipe inside is used as a purge line once the co2 reaches a certain depth.  

These are the suction cups I use:

[https://a.co/d/07tSOoNb](https://a.co/d/07tSOoNb)

DIY CO2 Recipe:  Use a standard 2L soda bottle, punch a hole in the top cap, attach 3/16 rigid tubing or use a length of 3/16 flexible airline and use some silicone to seal in place.  Add 2 cups of sugar and ½ Tsp of activated dry yeast.  Fill bottle half-way and shake to dissolve.  Leave unopened (!) for a few hours until the yeast starts doing its thing, then cap and attach to the diffuser.  Should produced co2 for a few weeks, when it stops, just mix up a new batch and repeat.