Taking care in Winter.

Winter tips and do’s and dont’s.


Winter is sometimes a confusing time for Koi keepers, it is full of different opinions and what to do and what not to do. It seems everywhere you go there is somebody saying something else…its hard.

I will attempt to give the best advice based on the clients and deal with now and over the past 20 years.

Koi are not a true coldwater fish, they have evolved in every way from the common carp they came from. They have been weakened by breeding. They do not have the same body as a wild carp, they have been fine tuned over many years. It’s like many dogs these days, they have been inter bred so much over the years they are a completely different animal to what they originally were. Wild Carp in lakes are far harder than our Koi, they don’t have an issue with parasites as such and being in the natural environment they are in, they tend to suffer less problems. If you did take a scrape from a wild carp in the height of summer then I am sure you would find pretty much all of them on there.

Sadly our Koi need treatments when parasites appear. Caught early enough of course it is easily sorted out.

Winter in the UK can be a terrible time for our Koi and a lot of people do not realise how tough it can be. I often talk to clients over the December/January months who happily tell me that their Koi are sat at the bottom ‘Hibernating’! Koi do NOT hibernate, its total nonsense. They have hit the bottom because they are freezing cold. The bottom is the warmest part of the pond so they sit there…and freeze. The metabolism slows right down because they are not getting food, because they are too cold to eat! The previous 2 winters have been some of the harsh I can ever remember, the fish losses are higher than I have ever seen before, this is all because of the cold. Koi will take a certain spell of cold weather but they will not take it much more than a few months at minus temperatures, they will die.

Now I am not saying that we should all be spending hundreds/thousands of pounds installing super duper heating systems but there are a few things we can do to help the Koi in these bad times.


Yes, of course in an ideal world we would all have gas boilers or electric heaters keeping our Koi at a steady 22C all winter and feeding them 3 times a day. The cost of the current gas/electric bills make this impossible for most of us, unfortunately. However gas/electric can be used and it can be used at a fraction of the cost of what people think.

If you have a way to cover your pond you will not realise how much warmer this will keep the pond, it will keep the chill off and save precious degrees which you would have lost without a cover. Poly sheeting like they use on conservatories is my favourite one, it can be made to fit most ponds and its pretty inexpensive. I am sure many window companies have a bit of it left over from each job, just ask. Bubble wrap is an old favourite too, tacked to a wooden frame will also save the precious degrees of heat.

There are many different opinions but 8C seems to be an acceptable temperature for people in the UK to keep their Koi. I would prefer 10C but again, its not an ideal world at the moment. If you had a heater, gas or electric and you had a good cover over the pond, then keeping your pond at 8C isn’t going to cost the earth. I had to the exact conversation with a client 20minutes before I started this article, his opinion was of ‘If I lose a fish, then Its cost me more not to heat my pond’ This gentleman has a 3000gallon pond and he has a 3KW Electric heater, he covers with poly sheets and last winter his extra electric to keep his pond at 8/9C was £120.00. Now I don’t think that’s bad at all. I ask the more established Koi Keepers who do not heat to go look in their shed and see what ‘Koi Gadgets’ you have bought at shops, or Koi Shows this year which will probably never be used that cost more than £120.00. Of course there is a lay out for a heater too. But please, at least get a cover over the pond, try and get it as tight fitting as possible, but please…cover it. Leave a small gap somewhere for escaping gases too.

Parasites are still active too, even at lower temperatures. If you leave the fish over winter without any kind of heat or cover then as soon as Spring is here the parasites will become very active and they will become active a long time before the fish recover from the winter. The fish need to feed to become healthy again, the immune system needs to be at full strength to be able to deal with and combat parasites, before you know it you have very sick fish.


This is one that truly stuns me each time I hear it, and I hear it a lot, even now!

“Shall I turn my filter and pump off over winter?”

I have no idea where this ever came from but I do know people who have done it and suffered terrible problems, I do not know why anyone would turn their filter and pump off, I really don’t. Any explanation please let me know because I have no clue!

Even in extreme temperatures bacteria will live, it will go to a very low count because there is not as much food going into the system and the fish are not producing as much waste as they would normally do, but its still there.

The biggest advantage of leaving your system running all season is that when Spring does finally appear in the UK, the filter is ready to go, it is not running at 100% but its running much better than adding a new filter to the pond, which if you turn yours off over winter is what you are doing when you turn it back on, starting all over again, crazy! Add a new filter to a pond full of fish and watch the readings go crazy, by the time you have sorted all the problems its Autumn!



Feeding at lower temperatures is a risky business. Nobody knows for sure about Koi. Wild Carp? Yes there are people who know exactly what temperatures wild carp will eat but like I have said, our Koi are not wild carp. Also opinions do vary.

My opinion on feeding is pretty simple, if you can keep your temperature at 10C then use a good quality Wheat Germ feed, at that temperature the fish will still be active so they will take a low protein diet. At temperatures less than 10C then the fish will head toward the bottom of the pond and this is when you have to be very careful about feeding. Feeding them now…could kill them!

When the Koi go to the bottom their bodies shut down, the metabolism slows to virtually nothing and the immune system suffers. So giving them a food with even a slighty high protein content could cause all kinds of problems. The fish would not be burning it off because they would still be sat at the bottom, the food would not be turned into waste anywhere near as fast as it should be and would sit inside the fish…and rot. This would cause internal problems with your Koi. Dropsy is becoming a common problem over winter and I am sure many reading this will have had a fish or two die with the disease in the colder periods. I visit many who have. However not feeding the fish through the colder periods can also cause problems when Spring hits, like I explained further up the article. There is something you can do though, and it really helps. A big mistake people make is throwing food in if we have a freak nice weekend, when we get 2 days of reasonable temperatures and the fish start to swim around, do not feed them. It will only last a couple of days until the freezing weather comes back again. Make sure we have stable weather before starting to feed again, remember the drop at nights.

Boiled pearl barley from the supermarket, very low in protein but will still give the fish enough to stay strong over the winter, don’t feed loads of it, just a small amount. It will even make the fish more active and stop them sitting on the bottom as much as they would. Its a great winter food for low temperatures and very cheap and is also freezable. I had customers using it for the first time last year and they could not believe the results and how different their Koi were from previous winters.


Yes, parasites will go much more inactive during colder months but they are still there. It is always best to get a scrape before the winter hits to make sure you have done your best to eradicate any parasite you may find. While they will not reproduce as much as in a nice hot temperature, they will still reproduce. If your Koi are sat at the bottom with everything shut down then the parasites will still cause them harm. Some treatments may not work at lower temperatures but there are some that will.


Now I am not trying to frighten anybody, I just want to explain what can happen over the winters. I am sure there are readers now saying “I’ve had fish for 20 years and never heated and never had a problem” yes I am sure you haven’t had any problems, but you could. New ponds and new Koi Keepers going through their first winters could have issues.

Just please follow the advice and it will help lower the risk of those winter problems you might get.

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