It is quite common for fish to slow down when the water gets cold and that leads to lower production as well. As a result of your fish slowing down, there will be fewer nitrates in your solution so you don’t want to overfeed. Its one of those things you’ll need to keep an eye on and make sure you’re feeding appropriately. For more cold weather aquaponics related question answers like fish response to colder temperature, appropriate temperature ranges for tilapia, best heaters for IBC aquaponic systems, visit http://verticalfoodblog.com/cold-weather-aquaponics-questions/