Doing as Catfishsam suggests will work, but really there is no reason not to just place the float type stock-tank de-icer right on top of the ice. As long as the temperature around the de-icer is 35°F or below, the built in thermostat clicks on and the heating element will heat up. At approximately 42°F the thermostat clicks off and the de-icer's element no longer heats up. The de-icer does not have to be in water to operate, as it works on temperature change and has no way of knowing whether it is in water or not. I have sat the same type of de-icer on top of ice to melt it many times without harming the de-icer in any way. If you are in doubt check the FAQ at the Farm Innovators Inc, site. They manufacture the de-icers. "Horton" P.S. abillus I hope it works out for you and it should. It will take a bit of time to go though that thickness of ice, so be patient. lol After you get the vent hole open I would suggest you use a timer to cut down on the power use. Only a small hole in the ice is necessary to allow for gasses to vent out of the pond. Remember don't let the element touch the liner as it could burn it. Take care.

In zone 7 you should not have any problem keeping a hole in the ice with a low wattage deicer. That's all you need. Just a small hole. I have a homemade deicer that uses two lightbulbs. I also have one that is 100 watts. It would probably do the job by itself but I'm hole obsessed once the snow starts. Remember that the higher the watts, the more expensive it is to operate. The instructions for a homemade deicer can be found by searching for Greg Bickal. Sandy

We'd probably need Cnetter to explain a ice melter safe for critters but it looks that it has glycol. And, glycol apparently changes to alcohol under the circumstances. So, that lowers the freezing point of the water and the ice melts. Safe paws . . . maybe drunk earthworms . . . Looks lots less scary than salt or cheap lawn fertilizer, for that matter. I see that various pet-safe brands sell for about $1.50/lb. I was in to the hardware store the other day and their salt must have been just as expensive! It's the Cl of the NaCl that represents some danger. Chlorine ain't very good for our environment in concentration. So, calcium chloride and potassium chloride don't represent an improvement, I suppose. The link below to the Mad Scientist has some info on various de-icers. I see that calcium magnesium acetate is considered the safest of all ice melting chemicals in regards to vegetation, concrete, and water but it is more than 30 times the cost of rock salt! So we are talking $45/lb retail!?! The Mad Scientist says a little about concrete corrosion. (Interesting that the article is written by a chemical engineer with the US Postal Service.) What is said is that we probably worry too much about sidewalk/driveway corrosion since freezing & thawing is far more damaging than most anything we could put on the concrete. Urea, in a purer form, is what airports use. Hmmmm . . . Seems to me that getting too high of a nitrogen component wouldn't be safe for the plants. digitS' Here is a link that might be useful: chemical ice melters - USPS

The best is none. Deicers have been a fav on web sites for the past few years but most ponds were fine without them for thousands of years before that and I suspect they'll be fine for thousands of years after the web and humans are gone. Gas exchange can be a problem, but so can cooling your water. An iced over in ground pond will have water at the bottom of 39F because that's the heaviest water. Water colder than 39F rises (warmer water doesn't always rise). But if you do anything to disturb this natural wonder the water will mix and all the water in the pond can be lower than 39F...even going below 32F (unlikely, but possible). Fish can survive in super cold water...for some period. Fish can survive in water with poor gas exchange...for some period. The question is which is the bigger risk for you? I've seen ponders in Sweden and Finland have gas exchange problems because their ponds ice over and stay iced over for really long periods. But even many of them don't use deicers. Instead they make sure the pond is clean before ice over. Rotting organic material causes the real gas exchange issue. O2 and CO2 are not the issue. Most ponds in the US for example ice over for short periods of time, maybe a month at a time before a warm spell melts everything. I kept ponds for years in upstate NY and we had periods where it didn't get above zero F for a few weeks. But then it's warm up and melted everything. IMO lower water temp is normally the bigger threat. Gas exchange is a threat but way over hyped online. I come to this conclusion just by looking at the millions of ponds in the world which ice over and the fish live. On the other hand dropping the water temp to 33-38F just can't be a good thing for fish. Most do survive deicers but lots don't. I've read cases of deicers breaking and electrocuting fish or catching fire and putting toxic chemicals into the water. And then many cases of fish dying but the owner saying it was because they didn't have a big enough deicer...and never ever taking a single water temp reading. My personal fav is when they say they had a bad cold spell and the deicer couldn't keep up and the pond froze over for a few days which killed all the fish. Every other pond on the planet can freeze over for months and be OK but not this pond. The concept of -30F air being pumped through the pond water driving the water temp down to maybe below 32F...no, that could be what killed the fish. Scaring people is more entertaining and many people do like to buy stuff (even while complaining the whole way) for their pond and "fixing" nature because they want to feel smarter or something. Just depends on what aspects of the hobby you enjoy.