How to Protect Your Fish From the Worst of Winter

By Ron Boedeker


Clean pond; remove all unattached organic debris from the bottom of the pond (leaves, algae, plant material). Do not pressure wash or brush the sides. The attached healthy algae is good for the pond & the fish.

Do frequent 15% water changes to dilute out the bad dissolved organics and help rebalance the minerals in the water and be sure to use dechlorinated the water. Do not do large water changes, it causes too big of a change in the water chemistry and stresses the fish.

Try to cover the pond to keep the leaves and wind-blown trash out. Transparent poly sheeting works best, shade cloth also works. Tenting the pond for winter is a great idea.

Inspect the fish closely for sores or wounds; you do not want sick fish in the pond going in to winter. If the fish are healthy and not showing any signs of parasites, do not treat the pond with anything. If the fish do have problems, you need to decide if it’s just one or two fish or a pond wide problem.

· If it’s just one or two fish with wounds, they need to be removed to a heated hospital tank and treated or the fish need to be euthanized. Do not leave sick fish in the pond.

· If the problem is pond wide then the pond needs to be treated.

· A koi’s immune system stops working at approximately 62 degrees water temperature, so anything lower than that the fish cannot heal itself properly. Any sores, wounds or other weaknesses will leave the fish vulnerable to a multitude of opportunistic parasites & bacteria.

· Treating the pond when the water is below 62 should be avoided unless it’s absolutely necessary. This both stresses the fish and kills off not only the bad things, but also most of the good things too. Below 50 most pond medicines will not work at all.

Feeding the fish:

· When the water temp drops below 68, stop feeding growth foods and move to 4 season foods, also start reducing the volume of the food the fish get.

· As the water temp gets to around 58, cut back even more on the volume of food. One feeding a day is plenty.

· Below 55, feed every third day or you can stop feeding completely until spring.

· Below 50, stop feeding completely until spring when the water temp stays above 50.

· Remember, it’s not only that the fish don’t need the food, but the pond biology has pretty much stopped too and it cannot process the fish waste & uneaten food.

Water plants:

· As the plants die back, cut the plants off just above the dirt and move the pots to a spot low enough in the water that the roots will not freeze solid and let them winter there. We do not want any dead plant material left in the pond.

Filters, pumps & piping:

· Fall is a good time to clean the filters, but be careful to not kill them off. The filter media should be gently rinsed in a pail of pond water not washed with chlorinated city water.

· Check the pumps & test the GFCI’s their plugged into.

· Clean the skimmer baskets frequently to keep them from plugging with leaves & debris.

· Everything that holds water needs to be protected from freezing.

· If you run your system all winter (which is our preferred option), your piping system should not freeze, unless you get caught in very cold weather and have a power outage.

· If you shut your system down, you need to be sure to drain & winterize all the piping & filters that hold water and remember to cover things so the rain & snow doesn’t refill them.


· Once winter sets in, it’s important to keep checking on the pond & fish. A lot of bad things can happen if no one is watching.

· Try to keep doing small water changes whenever the weather permits and keep removing any leaves or debris that settles in the pond.

· Try not to spook or disturb the fish anymore that is necessary.

· If the pond ices over, it’s very important to keep a breathing hole open. Do not let the pond freeze totally over. A solid ice cover will trap dissolved gases under the ice and kill the fish.

· Do not pound on the ice as the shock wave can kill the fish.

· Do not walk on the ice and risk breaking thru. Keep children, dogs & other pet off it too. If someone dos fall thru, call 911. Do not go on to the ice to try to rescue them.


· As the ice melts and the ponds begin warming up, watch for winter structural or piping damage. As frozen but broken pipes thaw, you can get a serious water leak.

· The fish will gradually start becoming more active and the parasites will be attacking any seriously weaken fish. If the fish went into winter strong & healthy, they should do just fine in the spring.

· The algae in the pond becomes active far earlier than the fish or plants do, so we get the dreaded Green Water or soup algae syndrome. You can let Mother Nature take her course or use some of the chemical products to combat soup algae or use UV lights to deal with it. Once the water clears a little, the string algae and larger plants should eliminate the soup algae and the water should clear.

· Do not be tempted to start feeding until the water temp is consistently above 50. Then reverse the fall schedule, but go slowly. The pond biology & nitrification bacteria have to get up to speed and you’re looking at 6 weeks or more depending on the weather.


· Ideally, you would have a well-built & properly maintained pond with a low fish stocking density and conservative feeding habits. Under these conditions, no chemical products are needed.

· But most of us don’t meet these conditions yet, so we use the chemical products to get us by.

· The most commonly used products are the ones to control algae and dissolve away the organic debris that accumulate in the pond. These so called sludge eating products work by breaking down the organics and converting them to byproducts that the algae cannot use for food, thereby starving it out. Of course, nothing works 100%, so the net effect is that ideally the algae will be controlled, but not go away completely.

· Killing the algae with an algaecide is not recommended. Controlling the algae with proper pond maintenance & plants is the preferred method.


· Salt is not needed in most ponds, but in some situations it can be useful.

· No plant needs salt. Some plants will tolerate low levels of salt, but most plants do not like salt, so we do not recommend salting the pond in the fall.

· If the fish are stressed or sick, they can benefit from low levels of salt (at the expense of the plants), and salt is not effective for parasites any more.


· The value of water changes cannot be over emphasized. We recommend 15% water changes as frequently as you can manage and they should be continued during the winter. The water changes will dilute down the bad things in the pond and help the good things.