Dear friends:
I have been into the koi-hobby for 2 years. My pond is about 25k gallons, 33X27 feet, half of it about 3 feet deep and the other half 7 feet. The water was very green when the winter 2018/19 came. I was told by RichToolBox that my biofilter was not working, and instead the green water algae took care of ammonia. I have an air-pump of 100 W going on 24/7/365. The koi survived an ammonia peak July 2019 when I accidentally killed off the green water algae. No other accident otherwise.
Summer 2017 (2 years ago) I bought 25 small cheap Israeli koi for testing, and 15 of them perished during the winter 2017/2018 outdoor in the pond. Last year I got some 15 bigger (15-20 inches) low-quality koi plus 4 better quality ones (1 nisai male and 3 tosai) just imported from Japan.
The koi was fine before the ice covered the pond December 2018. Of course, the air-pump left a hole in the ice-cover throughout the whole winter. Since I thought that the bigger koi would survive the long Swedish winter better, I have just expected to see them happy and nice again this spring.
Disaster:
But to my big horror many of the bigger koi died. Up to today I have taken out 8 floating, stinking dead big koi since the ice started to recess for about 2 weeks ago, and I suspect that there can be more dead koi on the bottom waiting to float up when the decomposition proceeds further.
I tested the water quality, and everything is OK. Zero NH3, NO2 and NO3, just as I expected as green water algae is excellent N-remover.
I donīt know if it is a coincidence, but none of the dead koi was the 4 imported from Japan last year, nor anyone which survived the winter 2017/18.
So I wonder:
Can it be that some koi are just better than others to survive the long (5 months, 4 months with ice-cover), cold winter? The guy who owned those 15 bigger, low-quality koi had always had them indoors, at least 10 C or 50 F in the water. Therefore those koi had probably never been selected for coldness. Swedish koi-friends of mine took their koi indoors too.
I plan to take the koi indoors in the future winters. But will it be good for their health if they never experience cold water? Maybe they need the cycle of the seasons: Warmth of the summer and coldness of the winter?
And, have Japanese tosai been kept outdoors in the winter in Japan, therefore all Japanese tosai I buy in Sweden have been winter-selected?
Thank you for your thoughts!
Simon