My wife and I have a 16 foot x 9 foot oval pond. Average depth is 13 inches, with the deepest part 31 inches. Last week the pond turned all green, with a slimy surface, and all our fish died, except 1. I took out 20 goldfish. I removed all the water from the pond, hosed it down really well, and refilled it. Started up the two pumps, which both lead to a waterfall bio-filter. What caused the green water, and slimy surface? When cleaning the pond I took out at least a foot of muck in the bottom of the pond. We have plants in the pond, including two water lilies, several pickerel rush, and other plants. All in pots. Someone, please help, as we want to restock the pond, but we are afraid to.