Hard to tell for sure from pic's...looks like brown algae.

It looks like mulm to me. It is defined as fish waste and other organic detritus that collects on the bottom of ponds over time.

Do you have a bottom drain? What type of vacuum do you use?

With rocks covering the liner it is much more difficult to vacuum the mulm out between the rocks.

No bottom drain. I use the Python vacuum, which uses my water hose pressure to suck up muck. It does a great job sucking this stuff up bc it's really light and almost "dusty" like in the water. About two weeks after it's gone (from my cleaning), it starts to reappear. It's very frustrating. Do you think I have fish overload? I only wanted 5 fish, but nature happens. :) I do plan on removing the 12 babies soon. What should I do/not do to keep this stuff in check?

My bottom does that ... only when it gets warm in the afternoon it will float (this summer when it was warm at night it'd float all the time) & I have to skim it out. I also vacuum it alot, it's quite frustrating & a PITA. I always thought it was some sort of algae so I'm quite interested in what people think.

I can give you a partial explanation. I have seen the same kind of deposits in plastic lined tanks, unlined tanks, slow moving natural streams and a large pond and waterfall built during the depression as well as most watergardens and ponds. It is a stew of dirt deposits from the air and rain, mulm, live algae and lots of dead algae, and a few minerals from all the rocks in the stream. You could get rid of it temporarally by sterilizing the pond and seriously scrubbing the rocks but that would mean getting rid of the fish and it will be back in 2 weeks anyway. It usually grows in sun more than shade but will occur there as well. It is more likely to form in still water or in spots that slow the water flow slightly which allows heavier debris to drop out of the flow. The stones you have in the waterfall that get such a pounding are actually acting as a sedimentation tank. Not a very efficient one since the rock layer is shallow but it will trap debris that will feed the algae and other biological stuff that lives in the pond water. You may be over feeding the biofilter. I have never added any bio active material to my five year old pond and rarely have a problem with the Skippy filter. I did buy some today that is designed to work below 50 degrees as an experiment but I expect I could do fine without it. I have no stones in my pond other than the ones that were knocked in by accident and the sides and bottom on the liner are virtually free of any algae growth or mulm but I know it is there. If you lift a planter that sits on one of the shelves or under a plastic crate the fishies use as a panic room you will find the deposits. The deposits occur anywhere the water flow is slowed. Excess food helps it along too. I expect to find similar deposits on the sides of pots as I pull them out for the winter. It will make them very slick or slimey to the touch. Make sure you are not overfeeding the fish, cut down on the microbes and remove some of the rocks at the lowest point. Mulm will collect there for you to vaccuum up and it will make you happy. Increase your filtration material. I use quilt batting on my skippy and a large prefilter on the pump for the UV light box. It won't get rid of all of it but it will reduce it. Sandy

Your pond looks really nice. This is a bottom drain (or absence of a bottom drain) problem. The rocks look great, but create many cleaning problems. We have nothing on the bottom of our pond. The mulm goes to the bottom drain then to the pressurized filter. We have never had to clean our pond in four years.

Sleepless or Sandy

I get it also ... on the bottom of my pond where there is no rocks (as well as every where else) & the water is not still.
I feed once a day & most times it's not every day, it might be every other day or every few days. It is in full sun ... the other descriptions don't fit me (I don't overfeed, no rocks where it collects (other areas, yes) water isn't still or slow moving (except in beachy area (where there is thin layer of rocks) & it's not there much)& yet I still get it.
Are you saying it's a filtration problem or am I misunderstanding? It's not a type of algae?

I had the same stuff last year in my first pond. This year we rebuilt the pond. We put in a skippy filter, and a bog filter and we put in a retrofit BD. I do not have that stuff in my pond this year. I do have it in the Bog filter. I don't know if my bog filter is truly a bog filter the way it is made. It has about a foot of gravel and is covered by about 12-18 inches of water and overflows back into the pond. The water is pulled from the bottom of the pond forced up through the gravel. This seems to collect all that stuff and the water going back into the pond is filtered. Glenda

Missy, what you're describing sounds like algae - the floating and sinking routine. It uses nutrients that plants might otherwise use to grow, so the more plants you have in there, the better. Not sure how many fish you have in comparison to your pond size, so the fish could be "fertilizing" your algae and making it robust. :)

AO's is almost like a pond dust, and I agree with Sandy's comment that cutting down on microbes (dry bacteria) will help greatly.

Brenda

Thanks ALL for the great info. Sounds like I need to use a finer grade filter (right now I'm using a filter pad that has gigantic holes in it and small debris can flow right through) and stop or slow down on the dry bacteria applications. I started the dry bacteria bc the algae was getting out of control. Now that the algae is gone, I'm getting this "mulm" or dusty debris on the bottom!

Thanks.

My water is clear & looks good (as can be seen at night) .... so now I just need to figure a way to get rid of this stuff.

If you are using algae killer, then the dead algae is contributing to the problem.

However, it is a pond and ponds aren't supposed to be sterile. It is normal and desirable for carpet algae to grow on the liner and it will on rocks too.

You also have a lot of fish and those koi are going to get huge in no time at all....

I've had a rocked pond for over ten years with no bottom drain that I don't vacumn more than a couple times a year but I expect carpet algae and I have a ton of plants and rarely feed my goldfish.