Oxygenating plants only oxygenate when the sun shines and at night they use up oxygen. In winter the decay products use up even more. Most oxygen in ponds comes from the atmosphere so some movement in the water would help. Plants are useful for absorbing waste products but you don't want too many. If you must feed the fish, feed as little as you can bear, otherwise the fish will get too big, waste products will build up, the pond will go green renering whatever fish you have invisible...before they die of lack of oxygen. Try this...take a small child armed with a pond net to a local river or pond get him/her to scoop around in the reeds and put the catch in you pond. At this time of the year you should catch lots of fish hatchlings/baby fish (you probably won't see them as they are very small) which will live happily in your pond and won't need looking after. The list of possible inhabitants includes ..minnow, roach, dace, bream, gudgeon, tench etc.