The stuff that collects at the bottom of a pond or lake is a combination of decaying leaves, fish waste, yard clippings, fish food, and anything else that may have washed or blown in in the most recent storm. Muck, sludge, mulm or whatever you call it, it’s brownish, slimy and slippery, and can smell bad or even poison your fish if it gets out of hand.
If your pond is fairly small and you enjoy some good old fashioned sweat equity, you could start off by using a muck shovel to remove most of it. A muck shovel is basically a mesh shovel that allows you to pick up the sludge while allowing the water to drain through - think of a colander or a tea strainer. This method is inexpensive and can allow you to get the problem under control quickly, although your filter will have to deal with any waste small enough to pass through the mesh. You’ll want to keep an eye on your water chemistry afterwards, too, since some anaerobic bacteria may have been lurking in the detritus. If the sweat of your brow doesn’t equal shining virtue in your book, a pond vac is a great option - it basically accomplishes the same thing as the much shovel, but all that’s required of you is to stand comfortably on the side of the pond and direct the nozzle along the bottom, the edges, or wherever you see “dirt” collecting. The vacuum will separate out all but the tiniest mucky particles and divert the remaining water to be returned to your pond. One huge advantage to this vs a mechanical siphon is that you won’t need to do a large water change at the end and your pond chemistry won’t take such a big hit. On the other hand, if you’re tackling a large pond or natural lake, your average pond vac won’t be up to the job. In this case, it’s best to call in a professional with heavy duty equipment. In cases where you’re rehabilitating a long-neglected pond or lake that has lost a lot of depth to accumulating sludge, dredging may be your best option, and if you’re game, there are even portable muck dredges available for rent.
