Adding some surface disturbance is important, but there are a couple of other things to point out here that I will mention momentarily. No mention is made of what filter you have, or if you have one. The filter return can normally be placed at either end so the water having passed through the filter is directed down the tank length with some surface disturbance. Even a sponge filter will do this. Additional bubbling devices should not be necessary in most cases, so start with the filter.
Having said that, the respiration rate of fish is the guide to oxygen; normal respiration is what you want to see except when fish are "excited," as they would be if feeding, spawning, "playing," etc. It takes a bit of experience to recognize normal/rapid respiration, but if there is rapid respiration the oxygen level is one of the first things to consider, that there is sufficient surface disturbance. Many other things will cause rapid respiration too, and in extreme cases fish remaining at or close to the surface and even gasping for air.
That brings me to the fish mentioned. Cories normally breach the surface on a regular basis. They do this because they swallow a gulp of air which passes to the hind gut where blood vessels assimilate oxygen; when the oxygen is used up, the breach the surface for a new "gulp" and expel the old air from the vent. The timing of this breaching is another clue--too often means something is not right with the water. Cories can sometimes reduce this breaching...I very rarely see my cories breach. But the point is that it may or may not be a sign of trouble.
Danios are active swimmers, meaning they need more oxygen, and they frequently remain in the upper level of the water column more than some other fish. This is another case where experience lets us work out how much is too much. I would have to see a video of these actions to be able to suggest it is or is not "normal," but it is just something to keep in mind.
Surface disturbance is not a bad thing though, so when all is said and done, more is not likely to cause issues. Juyst realize that fish behaviours take some getting used to to recognize.
