Giving an update to boost this. I am seeing them less and less. The few that remain are crafty, evading the suction of my pipette and the area they used to congregate around. However, I have discovered a new way to manually eliminate them. Since they are getting harder to take alive I just started smushing them. They don't seem to try as hard to get away from the pipette when I use it to crush them against the glass or into the sand. I am not even being swift and they just sit there and take it. Has this happened for anyone else?

 

I was doing okay just crushing them, but turning on the light at night revealed that there were many I wasn't seeing before. Today I tried a CO2 overdose with conditioned seltzer water. It was largely a success, with many enemy casualties. However, I must not have done it long enough because when I replenished the tank with oxygenated water I found two that had survived, and I am sure they are not the only ones. I killed a lot, I know because I picked out the floating bodies, but its pretty annoying to see that this could have been the final solution if I had just waited longer. I would recommend the CO2 overdose if you are struggling with dragonflies, just make sure to do it for long enough.

 

A lot of people in this situation end up gassing their tank with CO2 for a couple days non-stop. Have you tried over-saturating with CO2 for more than a day?

 

No, I only did it for like 45 minutes, but a bunch of unresponsive bodies were floating up to the top by the end of it. Does it really take days to asphyxiate all of them? I was just using seltzer water from a soda-stream, so I couldn't practically gas them nonstop like if I had a CO2 setup.