If you’re living in a region where temperatures get low during the winter months, you’re going to need a device to keep a hole melted in the middle of the pond through the winter. When a pond gets a layer of ice, a heater (or sometimes called deicer) is used to melt a small opening in the ice letting toxic gases out and oxygen to get into the pond. This is best done with an electrical pond heater.
However, the heater can greatly raise your electricity bill. Pond heaters pull electricity from 100 watts (for ponds up to 400 gallons) to 1500 watts (for ponds up to 1000-1500 gallons).
Alternatively, goldfish and koi can be taken indoors for the winter. They can be placed in a tank or a small children’s pool. But be careful with plastic pools, as plastic residue can be toxic to fish. A simple remedy for this would be to fill up the pool with clean water and add 1 tablespoon of salt for about gallon of water. Give time to settle for a few days, then rinse off and add fresh water. If the dissimilarity in pH levels is too big, the water can hurt the fish.
Pond heaters come in two designs, a submersible pond heater, and a floating pond heater. A floating heater will use electricity, oil, or natural gas for its power source. There are even some models with solar panels to power the heater. A floating heater uses hardly any energy and is essentially created not to heat the water, but instead to prevent ice from developing in the first place.
A floating pond heater can work both as a pond deicer and/or heater if your garden pond already has ice on the top. You can put the floating pond heater on top of the ice and it can thaw a hole through the ice, gradually melting all or most of the ice on the pond’s surface, depending on the size of the pond.
To get the most out of the koi pond heater make sure that you secure your system as much as possible from heat loss. The less heat you lose, the lower the electricity bill is and the more efficient your heating device is.
Quick note: Never use physical force to bust an opening in the ice. Fish are very delicate to shocks this can create and striking the ice can frighten or greatly injure fish.
Resources: https://pondpumps.guide/review-laguna-powerheat-pond-heater/