you are right if you were using saltwater that was just sitting in a bucket it would be low in oxygen but since you are pulling it out of the tank it has a lot of oxygen in there.
If you are worried and the fish seemed stressed you can agitate the top of the water with your hands or even an airstone. I wouldnt add a powerhead though it would be to much current for them to handle in that low of water volume. I would leave the airstone in though only for short time til the fish start to act normally again.
Mike