I have a small 80 gallon performed pond, had it for close to 20 years now, used to be in the back yard long time ago as part of five connecting ponds. Took it out a quite a while back when I made the bottom of three connecting "back yard" ponds bigger, now small performed pond is in the front yard, keep goldfish in that pond in the summer, and I enjoy it in the front, people like to walk and see pond out front, gets me out meeting and greeting all the neighbors. Kids come by in the summer and feed the fish worms and thats OK, cause that is why I have it out it there!
Sissy, clean is deceiving. You can have a spotless pond with no smelly guck at bottom, but ammonia and other problems. Then there can be the ponds that never gets cleaned like mine, no ammonia, not too much guck but leaves right now, and very healthy fish with no ammonia at all. My guess is that cattails kept water clean. Good bacteria live in the sludge and the smell is the pond digesting the waste. My sludge smells too in the spring. You have two choices when it comes to ponds... a pond that operates like a stomach, where the waste is digesting, hence the smelly sludge. When your food is digesting in your stomach it comes out smelly too! This is because bacteria causes the sludge to smell. Does not mean pond is unhealthy, as this same sludge can be found several feet deep in the cleanest lakes!
Then there is the "garbage can" pond. This pond can appear very clean, free of waste and sludge, rocks polished spotless, but ammonia bubbles building unseen, this is the garbage can pond. Because their is no good bacteria in a too clean pond, things rot and turn to ammonia, like the garbage can. That is probally why the fish were fine, seen this before over the years, a little neglected pond but has lots of healthy fish. Plants and bacteria were keeping the pond water clean even though there was lots of sludge at bottom of pond. I remember over 30 years ago I went to rescue fish from a much neglected pond. It was around October, leaves covered the ground and had settled into the little pond, and there was leaves and guck one foot thick at bottom of pond! There was no pumps or filter. Just lots of leaves everywhere. At first when I saw this little 40 gallon pond I thought there was no fish left, but low and behold at the bottom of the pond the leaves were moving! Took out several very fat cute healthy goldfish!
