Laguna PowerHeat 500-Watt Pond Heater for Outdoor Ponds with Fish – Thermostat-Controlled De-Icer, Safe for Plastic & Liner Ponds, Stainless Steel, 22 ft Cord – Keep Your Fish Safe in Winter for Koi, Goldfish & Aquatic Plants

We changed our pond winterization scheme this year. We have a relatively small pond (probably 400 gallons) fed by a waterfall, and in prior winters we kept the waterfall running even when most of the pond was frozen over. We'd run a bubbler but no heater. Our 5 koi have done well and grown large enough that our landscaper and "pond people" have all been nervous regarding whether we can keep everyone healthy under any winterization plan. To minimize the risks (to both fish and equipment) associated with running the waterfall all winter, we decided this year to try removing the waterfall pump and just relying on a combination of bubbler and pond heater. I bought the Laguna 500 W heater expecting that we'd see the pond freeze over but retain small open areas around the heater and bubbler.We're now halfway through winter and I've been surprised that the pond has not iced at all. The heater is warm enough to keep the entire pond in the high 30's (37? it's been a couple of weeks since I checked on a very cold day). We were down in single digits again last night with no sign of ice. The fish seem fine resting on/near the bottom and they seem appropriately active on warmer days (40's). The light on the heater has been red 99% of the time or more -- I don't think I've even seen it flash green once during the past month. So I agree with others who have taken off a star based on confusion regarding the significance of the red vs green light. I can't comment on the actual power usage -- given the complete lack of ice maybe this is heating more than necessary. But overall, we are very happy with the heater -- it has brought us a much more worry and maintenance free winter and the fish seem fine. If things stay this way for another 8 weeks we'll be delighted!Update -- the winter season completed a few months ago and I thought I'd update this. I've upgraded to 5 stars because we did make it through an awful winter without any pond trouble. It seemed to be in good shape when we removed it from the pond so I expect to get at least another season out of it. At some point during the winter, we disconnected the power for a few minutes and reconnected it; after that the lights worked much more as expected. Laguna could still do a better job explaining the lights on their website and in the product packaging (my instruction booklet is basically worthless). But that's a small complaint given that the device gave us just what we needed to get through a brutal winter with minimal effort required for the pond.Quick update August 2017: Just checking some old reviews and I thought it was worth mentioning that this heater continued to perform as described above through two additional winters. This past winter the lights did change from red to green in a manner that seems right; but we still had the entire pond clear of ice all winter. In that regard if efficient energy utilization is the most important thing to you I'll grant a yellow flag on this. But if the most important thing is preventing freezing without frying the fish -- what matters most to me -- this is just fine after 3 winters.