Question
Hello,
We have had this koi pond for a few years and have  never seen ther water black before? We clean it out 2 times a year and do not have hardly any sludge etc on the bottom just your regular alge.  What coulkd be causig the water to be black and will it hurt the fish?
Thank you so much for your help

Answer
Hello Lynn!
There could be various causes. First thing to do is test your water for the two most important items: pH and nitrite.
If those two values are okay (pH should be between 6.8 and 7.8 and nitrite must be 0), then adding an air pump to increase oxygen at the bottom of the pond should help.
Also, adding some beneficial bacteria should help to clear it up.
It is hard to say what is causing it without seeing the pond and its surroundings.