Oxyenation could be a problem if all you are getting is gas exchange from the trickle filter. It depends on the organic load. The bacteria in the wet/dry actually consume oxygen as they digest the organics in the water. Whether there is enough additional oxygen being added to meet the demand of the bacteria, lus re-oxygenate the tank is hard to know without testing. If there is algae bloom taking place that makes it even more difficult to figure out because the algae produces oxygen during the day, but uses it up at night when the lights are off. The extra power head will probably help by helping to move the water around and get some additional gas exchange at the water surface. If all your parameters are OK, my best guess is that you just had a stressed out or sick fish and it wasn't an oxygen problem.

What are you doing for water changes? Without a skimmer you need to be doing regular weekly water changes of at least 10-20% on a nano. There are a lot of things that can be in the water that we don't test for. Does your aquapod have a carbon filter? If so, how often do you change it? Those small filters don't hold a lot and can be depleted pretty quickly.