Articles to be uploaded, in the mean time few tips for winter pond fish care….
For ponds which are likely to freeze I would add salt. Please dissolve salt before adding to pond. Anything from 0.3% to 0.5% should be good. 0.3% is 3kg per 1000 litre of water. Salt should be added no more than 0.1% per day.
Minimal water movement and absolutely NO current in water (I’m sure its possible, almost) so fish won’t have to swim against current.
Air stone to prevent total ice forming on surface (salinity will help )
Extremely careful with feeding. I do not feed under 7 Celsius, from 8 to 15 Celsius only wheatgerm based food. (this is a controversial subject, often, I feed light protein food at over 12 Celsius)
Rain water will most likely lower PH to dangerous level in polluted/city areas so please check water parameters. PH buffer or bi carbonate of soda can be used to manage PH/KH
I never stop pump/filter but make sure water outlets from filter DO NOT create current/too much movement, if you do stop pump/filter, water change may become necessary unless lots of rain in which case you may have to add PH buffer/bi carbonate of soda.
Pond covers and small heaters (I use fish tank heater) are excellent to avoid lots of troubles in freezing condition.