Winter Pond Questions

I am new to ponding so have few dumb questions...

Live in zone 5 and have small pond...300 gallons and 15 to 16 inch
deep....residing in pond are 6 lovely gold fish 4 to 5 inches in
length..Temp is starting to get rather cold here...yesterday pond had 1/4
to 1/2 inch ice on it...I removed pump and filter laid some 2x4's over pond
and placed 4mil clear plastic sheeting over entire pond area held in place
with bricks. I also placed air pump in bucket with bubbler 6 to 8 inches
below surface. when I checked today ice was gone from pond area....Am
wondering if I need heater if so would low wattage heater work? If there is
warm spell is it ok to feed fish? If I feed fish would this create problem
with ammonia contamination? any advise appreciated...TIA