Fish Health – When the Ice Melts and the Water Warms!

Published on March 1, 2011

Photo by Philip Miller
Photo by Philip Miller

What happens to the fish in the spring? Why do they get sick and have such a tough time in this seasonal transition?

There is so much that is going on. Depending on where you live, the transition may be especially harsh. Let’s think about what these fish are going through. Fish are dependent on the temperature of their environment and get stressed when they are near their limits and when there are big temperature shifts. When the fish are close to freezing for much of the winter, they will be pretty stressed as this is close to a lethal temperature. Add a relatively rapid increase with a few warm days and now the fish are really getting stressed.

On top of this, the fish have not been fed for possibly months. So now we may be throwing a nutritional deficit on top of the temperature change. Another stress to add to he stack.

What else is happening? The biofilter has taken a pretty big hit and basically it is starting from scratch each spring. With the lack of efficient biofiltration, water quality can deteriorate quickly with the warming water. More stress!

Finally, all of those pathogens that are ever present in ponds come out of hibernation and start looking to cause trouble. In summer, when all is in balance, there is usually little problem with the nasties that live with our fish. Unstressed fish can fight off infection and generally stay healthy. But in the spring, with all of this stress, infections can and will occur.

What can we do to minimize all this stress? As far as temperature goes, it is often difficult to do anything. Some folks use heaters to offset low temperature effect, but this is often not practical. We can shade the ponds to avoid too rapid a temperature increase. If we were only dealing with temperature stress we may not have to worry so much as fish can tolerate some stress without getting sick. If we can then control some of the other things we can increase the odds of not having health issues.

There are many very high quality diets for koi on the market and with proper feeding they may help the fish through a difficult spring. A fish is only as strong as the food it is fed. I do not mean just in the spring, but all year long. A fish fed a high quality diet throughout the pond and water garden season is going to be in better condition for the winter and thus better in the spring. So encourage your customers to feed a quality diet.

Many feed producers provide choices intended for feeding during the winter and spring. Targeting the fish’s needs during this time can offset some nutritional stress. Stress generally reduces immune function and is what makes the fish more susceptible to disease. There are many immune boosters available, like – Glucan, which can be found in many brands of feed. Look to start with one of these feeds in the spring and it may help bolster a struggling immune system.

It is very important to carefully monitor water quality when filters are restarted. Keep a close eye on those ammonia and nitrite levels as well as alkalinity and pH. Remember that with alkalinity below 100 mg/l, biofilters will not operate at peak efficiency and they need peak efficiency in spring startup.

We can help things along a bit with a nitrifying bacteria boost product. There are several out there. I have had great success with one from Naturbac ( When the product is added to a brand new pond filter, no ammonia or nitrite spikes are noted after several days of introducing fish. Of course for poor water quality conditions, a good water change always helps.

I do like to point out that a functioning biofilter is so much more than just the nitrification process. There is a whole ecosystem in there that we will probably never understand and no two are alike.

Finally, we can try and reduce the number of organisms in the pond that can cause disease. As I mentioned above, stress plays a factor in the fish getting disease. Another factor is the type and number of disease organisms present. An outbreak often happens because one organism had the largest population and it took hold when the fish were weak. If we reduce the total number of disease causing organisms, then we may avoid problems altogether.

Prophylactic treatments may then be a good choice if you often experience problems in the spring. Try using a single treatment with a broad spectrum disease treatment like MinnFinn, or multiple applications of other products like malachite green and formalin mixtures or potassium permanganate.

Remember, fish can tolerate some stress without getting sick. If you can control some of the other stresses so the fish are only dealing with temperature stress, you can increase the odds of bringing the fish through with flying colors and ready for a successful pond season.

Kloubec Koi Farm

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