I agree with everyone above bout too much swan poo in the water, however, looking at your set up (which is beautiful and very well done) the mulch looks like it may be of the dyed variety. I know with a lot of our customers with ponds we will NOT use dyed mulch of any kind because it will turn the pond black after it rains and in some cases ie; heavy rain after mulch is applied, it is toxic to everything in the pond. All dyed mulch will leach the dye and chemicals used in the dye into the pond in varying degrees depending on the amount of rainfall. Usually after this occurs it will begin to smell foul and filters are of no use in any way to remove the color. If your mulch is dyed you may have to remove the mulch and use another type that has no dyes. I finished my own personal koi pond bout four years ago and used a dyed mulch around it. My slopes also drained away from the pond, however, after the first rain it became black and was filled with floating koi. I figured that it was a one time deal so did nothing but clean the pond and start over with fish. The next time it rained it turned black again but not quite as bad. I removed the dyed mulch after fussing with this for a year and have not had a problem since. We also had this problem with a number of customers that have used dyed mulch.
