There are bettas outdoors mixed with all the other fish? Not the best combination and pond temps in this area are rather cool for tropicals right now. A few more weeks, and it will be too cold for a good number of these fish.
Exactly what are you feeding everyone and how much/how often? With all of these juvenile fish, you should be feeding them 3x a day minimum. The ones that will overwinter outdoors should be fattening up now as soon it will be too cold to feed them (50f pond temp, not air temp) and they will not fed again until spring. The amount of food necessary to support all of these fish for proper growth would double your nitrates in a day easily unless your doing big water changes daily.