I have been told by a number of other pond owners and dealers that there is a problem with pond water colour this spring, turning into a dark brown tea. In speaking with them we have come up with the probable cause - POLLEN. This spring has had an extraordinary amount of tree pollen in the air - a brownish green color. On some days everything is coated with a thick layer. We generally feel that it should clear up with the end of the tree pollen season pretty soon. One way to lessen the amount of the colour would be to do a water change. I wouldn't do more than about 1/4 - 1/3 of the total volume cleaning the brown residue out as you empty. It's a very good idea to use the run off water to fertilize your garden which is very high in nutrients that plants just love. I use two 25 foot sump pump 2.25" hoses, inexpensive at Home Depot, to reach most of my garden. Remember to add pond salt back into the water all around the edges. It's good for the health of the fish.
My Favorite pond supply is Pond Pro Aquatics in Jobstown, NJ. The owner Don has quite a great setup. They absolutely have the most pond fish that I have yet to encounter. Mostly KOI but also carry a large selection of Comets, Shubunkins, Tadpoles and Snails with a large supply of aquatic plants. The free range Chickens make it an interesting visit. The shop is located on his father Don Sr.'s family farm in the middle of farm country in NJ. A very nice drive out there past horse and goat farms. for information - 609-723-4220