I found that having my waterfall running, where the water was going 2-3' above ground level, made the water colder quicker. I leave my bogs running all year long, though. It's my thought that since the water is circulating to the bottom of the bog, about 12-18" below ground level, it's pulling warmth from the ground and circulating it into the water, thus warming the water. My koi pond bog got an "ice jamb" along the edge, causing water to flow over the edge, and thank goodness I caught it early and shut off the pump. So, for the last month, the koi pond had no moving water, except for my air bubbler, and got iced over for part of that time. The goldfish pond, on the other hand, had the large bog running all the time. The koi pond froze over quicker and much thicker than the goldfish pond when we had temps down to near zero. We have been warm, cold, warm, frigid, warm .... all winter long! Currently going back down to teens and 20's for the next 5 days, so the ponds will freeze again. Got the koi pond bog running again on Sunday, though. The koi pond will be frozen 2-3 days before the goldfish pond will freeze. I also have my stream running into the goldfish pond.