I have seen water turn black, and I believe it was due to a combination of tannic acid (from oak leaves, bark or galls) and some source of iron. Leaves just happen. All you need is an iron source and you have black ink.

Here is a link that might be useful: how to make ink with oak leaves and iron

Go to the forum on www.pondkoi.com as they are a good source for your type of question....it sounds like a good challange for someone !!!! Good luck

I don't know how long the animal was there or how large but the rotting flesh is processing the same as would excess food and fish poop in the absence of enough oxygen. The water needs filtration, lots of Activated Carbon and an exchange of fresh water. If it was me I would pump out ALL of the water and start over since it is a waste of good filtration. It would be a lot cheaper to replace the water than it would be to use up all that filter material. Additional oxygen will encourage good microbes to process all that contamination. It comes with bubblers, a good biological filter or a fountain. The odor that is certainly going to be trapped in the liner is going to be a real problem. Some of it can be removed with the Activated Carbon but I don't know if you can ever get rid of all of it. Sandy

If the blackness is caused by organic stuff due to decay, even a bubbler will help a lot to speed up the decay leading to eventual clearing. A biofilter would be ideal. You should be able to tell by the awful smell of the water if the blackness is from decay. If the black is due to tannins, activated charcoal will remove it.

I've been thinking about this all evening and it occurs to me that there are cleaning products that might help. I don't know what you would "search" for but I think companies that clean up locations where bodies have decomposed must use some sort of enzyme cleaner. I have used an enzyme cleaner to get rid of pet waste that was left on a carpet. It removed the odor when nothing else would and it was really old. Of course there were some things it didn't work on like bare wood and some plastics. Sandy

Before you go adding enzymes and charcoal, check the pH and kH. If the pH has gone south, you might get a lot of mileage out adding some baking soda. Knocking down the acidity might help with the black color and the smell as well. If it is still bad after checking and adjusting the kH, then you might consider some of the other suggestions.

Is this a dirt-bottomed pond or concrete or liner?

if it were me, i would do nothing until springtoo cold and much of the biological fitration (microbes) are not active and anything you do will be slower than when it warms up. save the money on buying chemicals and things to throw at it. nature has a way of taking care of this by itself and you don't seem to have any pressing need (no fish to save or parties to prepare for). cold+organic matter+still water= stink. the black muck at the bottom of the clearest pond smells like like death with a hint of sulfur. if you touch it no amount of scrubbing will get the sell off your skin. the anaerobic bacteria is about the most foul thing on Gods green earth, but it had a purpose. if you mist deal with it now, i would just drain it for the winter and forget about it until spring. use this time to read up on ponding and to plan your filtration to keep it from happening again. winter is a great time to buy pond equipment on sale.

when spring comes if the smell and color persist and the pond is not too large, buy a sump pump and water the lawn. put on some wellies and rubber gloves and shovel and clean the muck out if you can. then fill it back up and start clean. starting fron scratch is the easiest solution sometimes. i would recommend going all-out if you want a crystal clear pond. get the biggest filter you can afford (i like overkill, and you will have to clean it less). there are several types of systems and theories as to which is best and easiest and they are all right for the right person. i think it a personal choice. my system is as follows. skimmer with a screen basket (removes most of the leaves before they sink) and a bottom drain with screen to pull water off the bottom of the pond. i use a "solids" pump that can pass or pulverizes an snail or leaf without clogging. then i have a pressure filter that is 3x over sized. i like pressure filters as i can clean them without disturbing the pond or getting wet. if you go this route make sure it has a back flow capability with agitation for easy cleaning. i also have a UV sterilizer which nukes any planktonic algae and other little nasties that pass through the filter. i then return part of the water to the bottom of the pond (to circulate any dead water) and the rest comes out of a tiered fountain which i have planted with water cress and parrot feather (they act as a bog filter). the fountain also aerates the water. the pond is also heavily stocked with plants. the water is 30" deep which keeps the temp more stable than a shallow pond. the water is clear and odorless. this sounds like more information than is needed, but when i divide the lilies in spring or am working on the fountain or something that requires me to be in the water messing up the funk on the bottom of the pond the water turns black and smells terrible like yoursÂand this is a healthy stable concrete pond that has never crashed, fouled or even had a fish mortality. a few hours of circulation and the smell is gone and the pond is almost clear but clouded. by the next morning everything as crystal clear again. good mechanical and bilogical filtration with a bit of aeration will solve most all problems without any chemicals.

i think what you are experiencing is just natural decomposition of all the stuff that was living in and around it during the summer (mostly plant matter). clean out the gunk and then work on some good filtration. if you want to keep the water calm, a simple slow moving bog filter would do wonders for the water quality. you can also plant the edges and use floating plants to filter still water. pond stores also sell biological cultures you could add for a jump start if needed, but again it won't do much during the winter. many pet stores will do free to cheap water tests that can tell you your PH. if it is too acidic, that should be buffered. concrete ponds rarely have this problem because the lime in the concrete acts as a buffer. i have heard some people use concrete blocks as a constant buffer in plastic lined ponds. you will want to rule out acidity or correct it before adding fish.

good luck.

Does this pond have rocks on the bottom?

If it is indeed an empty pond with no moving water, I'd consider draining it and refilling. It will be less frustrating and less expensive in the long run than loosing fish later. At the very least I'd do some major water changes.

I'd also think about netting it to prevent it from happening again.

I vote for either total draining or periodic water changes until the water clears, especially if a dead animal was involved.