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905-715-2447 Summer Hours: Open Wednesday to Saturday 10:00 am - 5:oo pm 3301 Sideroad 10, Bradford, ON Canada

Important Koi Pond Temperatures in Spring

Important Pond Temperatures in Spring

Spring is a critical season for your pond, especially with regards to pond temperatures and koi and goldfish. After a long winter of cold water temperatures, not eating and sometime less than ideal water quality, koi and goldfish are susceptible to parasites and bacteria. During the winter months when water temperatures are below 40 F (4.5 C), koi and goldfishes’ metabolisms slow right down and their immune systems are dormant. Luckily, pretty much everything else in the pond is dormant during the winter, including the ‘bad’ disease causing bacteria such as Aeromonas / Pseudomonas as well as many common parasites.

Important Koi Pond Temperatures

Aeromonas Alley

Once the ice melts from the pond and the water begins to warm up in the spring, begins a critical and potentially dangerous time period for fish. This spring ‘danger zone’ is often referred to as ‘Aeromonas Alley’ which is when the pond temperatures are between 50 F (10 C)and 65 F (18 C). As the pond temperature reaches 50 F (10 C) these bad bacteria begin to grow and multiply in the pond much faster than the fishes’ immune system. In fact fishes’ immune systems are only working at 50% efficiency when the water temperature is 65 F (18 C). To make things worse, many common parasites such as Costia, Chilodinella, Trichodina and Flukes also begin to rapidly grow at the same time.

If you cleaned your pond properly last fall, this greatly increases the chances that your fish will be fine. Many parasites and bacteria thrive in ponds with lots of sludge and organic matter at the left in the bottom.

During the spring, water quality can also be a problem. The good bacteria that live your pond and filter (Nitrosomonas and Nitrobacter) that break down harmful ammonia and nitrites multiply very slowly compared to other bacteria, especially in cool water. It is always a good idea to feed your fish sparingly in the spring when temperatures are cool and monitor water quality (ammonia, nitrite and pH) with a water quality test kit.

Watch Fish Closely for Signs of Stress

If you notice any signs of stress or unusual behaviour with your fish, try to take a close look at them. If any of your fish have a parasite or bacterial infection, treat immediately. If you catch an infection at an early stage, there is a much greater chance of successfully treating it. Some signs of bacterial infection are – ulcers (red sores), red streaks in in fins and on body, fin rot or mouth rot. Signs of parasite infections are – white spots on fins and body, a greyish film patches on body, excess mucous/slime coat on fish, fins clamped close to body. Treat with KnockOut, Pond Rid Ich or Microbe Lift Broad Spectrum Disease Control, these are safe to use in water temperatures are above 50 F.

Important Pond Temperatures

  • 40 F (4.5 C) and below – very little activity in the pond
  • 48 F (9 C) – Aeromonas – ulcer causing bacteria become active and multiply
  • 50 F (10 C) – Costia – a common pond parasite begins to reproduce quickly. Flukes are slowly reproducing.
  • 50 F (10 C) – Nitrosomonas a good bacteria in your pond and filter that convert Ammonia into Nitrite become active
  • 50-55 F(10-13 C) – Koi and goldfish should have ‘awakened’ and begin to feed. Their immune system is starting to work again. Start using cold water bacteria to jump seed filters with good bacteria
  • 58 F (14.5 C)– Nitrobacter the good bacteria that converts Nitrite into Nitrate in your pond is becoming active
  • 60-64 F (15-17.5 C)– Other parasites become active (Ich, Trichodina and Chillodonella)
  • 65 F (18 C)– Fishes’ immune systems are getting stronger – up to 50% of full strength and can fight off most typical bacterial and parasites in the pond if water quality is good.


Usually, once your pond warms up above 65 F, the chances  of your fish becoming sick is much reduced. As long as proper pond maintenance is performed and water quality is maintained, you should enjoy a trouble free pod season.

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