Question
I have a man-made pond approx. 30 x 100 ft. with water from 2 ft to 12 ft deep.  It's fed by an overflow of an adjacent brook thru a rock dam and has no outlet, and also has about 200  goldfish and one coi.  The problem is that the water smells like an open cess pool.  We recently placed a pump in it to circulate the water, but it doesn't seem to alleviate the smell.  Is my pond dying??? Thanks

Answer
I have a few large ponds and one almost the same size with about 200 gold fish, 20 koi, and lots of other life like frogs, tadpoles, snakes, and newts. My pond runs very smooth and is very healthy.

I have an outlet. I made a skimmer like system outlet. So my pond is fed clean water constantly. I think it is important to have some sort of filtration or an outlet. If fish die and life dies and settles in the pond it will decay and smell like death (really bad odor).

My pond is gravity fed by a spring fed creek, I dug a ditch or trench from my ponds far side wall (near the other side of the spring/creek I pull my water from) and buried a PVC pipe to let water out. So I have the same equal amount of water coming in as going out. The water flow is important and keeps it from being stagnate.

If I were in your shoes I build something like this and drain the pond low, and try to clean out some decay then let your new system do the work from then on. Adding a large carp to the pond helped me with some bottom of the pond clean-up. I mean a large common carp found in lakes. Hope this help a little.