On my two ponds, leaves and so forth in the goldfish pond, but once I got that out, that was pretty much it. Almost no black muck in the goldfish pond. Koi pond had almost no leaves or debris, but more black muck. And, I have learned that early in the spring, when water if just getting over being frozen, it goes through green or brown murky water, not very clear, but I don't worry anymore. Once the plants start growing in the bog and lily pots, the water starts clearing. It's amazing how that works. I can see to the bottom of the koi pond, not crystal clear, but I can see all the fish. Goldfish pond has been clear year round. Oh, and koi pond has lots of spongy looking algae that floats on warm sunny days, goldfish pond has none. I'd like to know why that is ... must be difference in koi poo and goldfish poo. Also, more koi on that larger pond than goldfish in the smaller pond, since I lost about half of my goldfish last summer.