Do I need to heat and or deice my koi / goldfish pond in winter?

Should I deice my koi pond?

I get quite a few inquiries about the need to heat koi ponds in winter to keep them from freezing over.  There seems to be some confusion out there about what is needed.

  • Does this need to be done to protect the fish?
  • Do I have to keep all the ice off the pond all winter?
  • Is it ever safe for the pond to be iced over and if so how long?
  • If my pond needs to be deiced, how can I do this?

Generally ice on a koi pond for a week or two is not a problem for the fish.  At this time of year, the koi have already gone into hibernation and are not feeding.  This means that their metabolism has greatly slowed down and they are producing very little waste.  Plant life has died back and vegetative material has hopefully been removed from the pond so there is little to decompose.   Even if there is some vegetative material, with the water temperatures in the 30s and 40s, very little bacterial activity is occurring.  So, overall, the biological processes in the pond have slowed down to a crawl.  This means if any toxics are being formed, they are doing so at a very slow rate.  Therefore, a pond that is totally iced over is not in immediate danger of toxic poisoning to the fish.

Ice over does become a problem when this condition continues for more than a few weeks without respite.  Even though being produced at a very low rate, toxics can accumulate over time and stress or kill fish.  Unfortunately this stress is hard to detect while the fish are parked in the deepest part of the pond.

How will you know if you are having a winter fish kill? You may not know immediately as fish that die in cold water frequently do not immediately float up to the surface below the ice.  More than likely you will not know until the ice melts and the water warms just enough for bacterial activity to start decomposing the dead fish.  At this time, the bodies will bloat and be carried to the surface.  Not a pretty site.

Even if the toxics are not severe enough to kill the fish immediately, you may lose them later in the season.  Stressed fish this time of year typically show up when the ice melts.  At that time when the rest of the fish are beginning to surface on warm sunny days, the stressed fish will either hang by themselves at the bottom or if in dire straits, will gulp for air continuously at the surface.  By then the damage has been done

What to do?   It is not necessary to keep the pond all ice free all winter.  It is necessary to open some open water at least once every 10 days.   So, if you live in an area that has periodic icing in the winter, you probably do not need to take any action at all.  If you live in an area where ponds routinely freeze hard you can either periodically thaw your pond out as described below or invest in a deicer.

Deicing without a deicer: To vent toxics, you need only to open a spot a foot or two across in a backyard sized pond and keep it open for a day or two.  Here is what NOT to do.  Do not open the ice by pounding on it with a hammer or throwing rocks on it.  The vibrations from such actions will throw your poor disoriented fish into a frenzy.  Remember, your goal to provide a stress free winter for these guys.   What you can do aside from buying a deicer is:

1.      Hang a 100 watt light bulb about 6 inches over the ice, one with a utility deflector and cage, if you have one, to focus the heat down.  Leave it on until there is a hole in the ice at least a foot wide.  Yes, be careful.  Don’t climb out on the ice to hang the light where you will have to retrieve it later over thin ice.  Dah!  Just locate it over the edge of the pond.  Secure it firmly so it has no chance of blowing into the pond.  Plug it into a GFIC receptacle so if a blizzard wind comes along and blows it into the pond, your fish will not be shocked. (GFICs interrupt the power immediately when shorted and are required on all outdoor receptacles)

2.      OR, Pour a bucket or two of hot water over the surface of the pond until you have a foot wide hole in the ice.  Reapply enough to keep the whole open for a day.  Then let it freeze over again.

Deicers:  Pond deicers are made to be plopped into the pond before it freezes over.  They are made to be submerged so electrical shocks and shorts are not an issue unless you have curious pets that chew wiring.  These deicers are cheap to buy,(less than $50) but at 300 to 1500 watts, somewhat pricey to operate.

(At a typical $.12 per kilowatt price, a 1500 watt heater run continuously would cost about $4 per day to operate)

The better ones have thermostats so they are not running constantly.  A good substitute for a pond heater is a stock tank (horse trough) deicer.  They are built almost identically and can be purchased at most farm supply places like TSC at a lower price than a pond deicer.

In conclusion, only deice if your pond freezes solid for more than two weeks at a time.  Then open the ice gently at least a foot wide for a day, about every 10 days.


  1. Excellent pond blog!

    Unfortunately a lot of beginners in the pond hobby often lose fish at winter time as they fail to leave a whole open in the ice. With the help of your blog, you’ll prevent many people from having this happen to them.

    Keep up the great work! You have a very informative pond blog.


    Jamie Boyle
    The Goldfish Guy

  2. Dee Prather says:

    I have a pond de-icer which is working well to keep an area of the pond clear of ice. I’ve noticed that there is a ring of foam collecting around the floating de-icer. Is this normal?