I used a regular 500mL plastic water bottle to make a DIY shrimp trap - that is, cutting off the top 1/3 of the bottle, invert and lay it on top of the remaining 2/3. I put some shrimp food inside the bottle to lure the shrimp inside.
I returned after 3 hours and found the DIY shrimp trap full of shrimp, as expected. To my horror, there were also inside the trap 2 otos, 6 pygmy cories, and 1 chili rasbora - all laid belly up, all dead!
No shrimp was dead. The remaining cories and rasboras outside the trap were alive and healthy.
I have used this type of DIY shrimp trap before without any fish getting trapped inside, so never experienced this kind of catastrophic event. I could not come up with a good reason: I guess the fish died from a lack of oxygen inside the bottle because they were crammed with ~ 50 juvie red cherry shrimp inside a water bottle; there was also the unconsumed food, the wastes from shrimp and fish, and the CO2 expelled from their mouths. But 3 hours seemed really short for them to run out of oxygen, especially considering that the bottle was open to the water column of 20G tank.
I wonder if anyone else had similar experience and possibly better explanation for fish deaths inside a shrimp trap.