Well my little pond after almost 10 years I had a ph crash and a nitrite explosion from hell. The only thing I did different this year is I didn't totally clean out and drain the BIO which is 100 gal stock tank with a 3600 gal a hr pump. Over the years I have left the system up and running year round after I clean the bio at least yearly, usually in July when I'm home on vacation and the weather permits. I have read over and over and over that this type of bio system doesn't have to be cleaned ever, You don't even so much as have to touch it, that the dirtier it gets, the better is suppose to work, well maybe thats the case if you live in California, Florida, or even Hawaii were the tempature doesn't get below 50. But if you live where it does get cold, well thats Bull Crap and please no one try it like I made the mistake in doing this year. I thought this year i'll not clean it and the first night it got down to 17 degrees is when my problems started. Lost a total of 7 koi between 3 and 10 years old. Tested the water and the PH was well below 4, thats as low as the color test will go and 4 is the deadly danager zone. Tested the nitrites and that was sky high. Makes me just sick to think about my fishies suffuring in that posioned pond water. The weather finally aloud me to drain the pond almost emempt and refill with fresh tap water with water neutralizer and a bottle of stress coat and 2.5 pounds of sea salt and the fishes that didn't die are finally back to swimming and enjoying their little pond world again. I learned my lession for the year. Don't trust everything you read. I'm mad at myself the most because I have been ponding for almost ten years and should have know what had been working for me is what i should have done in the first place and that was to drain and clean the bio filter anually. Never again will I neglect my ponding duties and put my fishes as risk. I counted 12 this morning and will see how they totally recover over the winter months. Sandy