oxygen question
 

I'm running a 15G tank skimmerless doing weekly water changes. Will the oxygen levels be ok with a powerhead circulating the water?

I would think a powerhead would be sufficient, as long as it is pointed upward to stir up the water on the surface where air exchange happens.

I have a small koralia stirring up the surface for gas exchange. I'm not sure how oxygen enters the water without any kind of bubbler or skimmer.

Surface agitation is quite more effective in delivering O2 in a tank and taking off CO2. Just as long as the surface breaks a bit it should be good.

As the others stated, surface aggitation is what you need. As long as you can get some good ripples in the water, the disruption in the surface tension of the water surface will do the job.

I didn't realize how much oxygen could enter with a little water flow.