Koi Fish Pond SuperCooling: What Is That?

Koi Fish Pond SuperCooling: What Is That?

SuperCooling, Common and Preventable


Supercooling – When you want a cup of coffee to lose heat, the fastest way is to pour it through the air from cup-to-cup until it has lost it’s heat.  

And in the same way a waterfall and fountain can propel your water through a thin-phase causing a convergence of air and water temperatures. This means that by day, your fish will warm faster because the water can pick up heat from both sun and air, but by night, the pond plunges down as the water gives up it’s heat passing through the water feature.

Here’s how the typical case appears. This is a true, documented case:

“This is real strange again. On Thursday a fish was laying on its side at the bottom of pond. For all appearances he was dead. But when I got the net out to retrieve him, he swam away. This is a 12″ Kohaku that is in my main pond outside….”

“An hour later he was back on his side so we netted him out and I did slides, 2 from gills and 2 from body. Nothing! The ammonia, nitrite and nitrate are all zero. Ph was 8.2 which is normal for me. Water temps was 38 degrees. …”

“I put him in a 20 gallon tank inside and let the water warm up from the 38 degrees to about 62 on its own. He swims upright and does not lay on his side at all now that he is inside. There are no marks, ulcers etc anywhere on his body. Gills are bright red in color. He acts totally normal now that he is inside. Swimming and staying upright. My hubby said maybe he just wanted to come in from the cold so that is why he was playing dead. Surely that cannot be the answer!”

“We are having and been having big temp swings the past few weeks. I have been feeding Impact every few days if the water temperature warrants feeding….”

“This morning the outside water temp was 42 tonight at 6 PM it was 53.”

“The water fountain (3900 cal pump) shoots the water up 10 feet so it is not an oxygen problem…”

“The thing that bothers me is the 4 fish I lost in March did the same thing, they just laid on their side and moved when you tried to net them. They eventually died.”


“There’s the whole answer:

‘We are having and been having big temp swings the past few weeks. I have been feeding Impact every few days if the water temperature warrants feeding…..The water fountain (3900 cal pump) shoots the water up 10 feet…”

When fish are rapidly chilled they lay over. Laying over is seldom  more than a clinical manifestation of shock/severe stress.
When brought and slowly warmed up, they usually recover, but they ought to be injected with antibiotics at least once, or they will die a week after warming up. This is because the bacteria will traverse the gut when the fish is nearly frozen and proceed to kill it later.

Do not feed when you think, or even imagine, there’s even a 2% chance of dropping below fifty degrees (F) within three days of feeding..

This fish should survive, but you should evaluate for things which may cause “supercooling”. Waterfalls and fountains can make water and air temperatures parallel, which means that when the air temperatures plunge and your waterfall is running, so shall the water temps fall, and when air temperatures warm by day, if there’s a fountain or waterfall running, so shall the water be warmed, and thence when the air temps fall by night, so shall the water temperatures fall, and thence the fishes will layeth over….I need a cup of coffee…it’s early.”

Best Regards,

“Doc Johnson”


  • Maintain water circulation at all times but when the temperatures are dropping, the circulation should be gentle and minimally break the water surface.
  • To keep water moving under the surface, disturb the top HALF of the pond water only. For example, do not put the circulating pump on the pond BOTTOM – allow the bottom depths to remain minimally disturbed when it’s cold out. Circulators could be halfway down in the water column.
  • Do not feed when you think, or even imagine, there’s even a 2% chance of dropping below fifty degrees (F) within three days of feeding.. Exceptions would include those areas (Portland) where water temps do drop below fifty but never below forty..
  • When temperatures are climbing, you can employ any feature desired.
  • When ice is a threat, you may maintain a hole in the ice for gas exchange with a cattle trough heater (35$) which will NOT disturb pond hydrology or cause turnover in the water layers.
+ “Supercooling theory can be of some benefit if strategically used in the cool of the night or the heat of the day depending upon what your water temperature goal is. Running your pumps during the day can absorb heat and turning them off at night can conserve it.” Doc Johnson
+ Note From Doc Johnson
In the winter, I just run a circulating pump for underwater circulation, and I return my filter under water as well to minimize surface disruption. I do not remove water from the deepest part of the pond, preferring instead to let that water remain still for fish rest.” Doc Johnson

Koi Fish Pond SuperCooling: What Is That?

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