if i had to make a guess, i would say that your fish is upside down because he is in shock due to the incredible amount of stress your fish were just subjected to.
since you asked for a solution to your problem, i'll do my best to give you one...
for starters, you need a bigger tank, a much bigger tank... minimum 75 gallons for that many natts, having that many fish in that small of a tank is a horrible idea. you also need to take that oscar out of that tank immediately, it does not belong in the tank with your natts, all they are doing is creating more stress for each other. those are the two biggest things you need to fix, and they need to be fixed right away.
you also need to just leave your fish in your tank, netting them out and placing them in buckets during tank maintenance is totally unnecessary, don't even take them out to take pictures or to measure them, it just puts way too much stress on them.