There are a few good videos on YouTube with some folks that maintain outdoor ponds. It's something we've been trying to figure out on a small scale, so I will likely use a heater for my planter/ponds.
My greens don't get nearly as excited about most foods like my guppies, but they LOVE live baby brine, and Easy-Fry Food. They eat some flakes too, but they pick at it rather than gobble it.