Sorry to hear about your loss.
Do you have rocks on the bottom? Do you regularly vacuum the bottom?
Years ago in my first pond after quite a few years, I had something similar happen. I lost almost all my fish.
I had not cleaned and vacuumed my pond as often as I should have.
With my second pond, I clean my bio filters with pond water at least once a month and do a twice yearly vacuuming.
I have two bottom drains that help quite a bit but due to irregular sides, vacuuming takes care of the rest.

Thanks for the reply. No I don't have rocks in the pond at all. The only rock I have in the pond is a concrete brick, the kind used to build a foundation that has been in the pond for almost the ten years I have had the pond setup. The purpose of the brick is to keep the pump up off the very bottom of the pond. There's no way the concrete brick had anything to do with the problem since its been in the pond for all these years. The pond is around 1200 gals and is half in the ground and half above ground. I cover the pond with shade cloth and there is no ground run off or leaves that get in the pond at all. The liner has a nice fuzzy type algae on it other than that the liner is as clean as the day I put it in. No yucky muck on the bottom at all. The pump was taking all the yucky stuff right into the bio I'm sure. I haven't cleaned the BIO yet but I have it disconnected from the pump so what's in the BIO isn't cycling back into the pond at this time since all the test showed the water in the pond and bio was deadly. I'll take a picture of the bio tank tomorrow to show you whats in the tank before I drain the tank and re-hook up the pump. Right now I just have my smaller pump under the water surface in the pond for Oxygenation and for keeping the water from freezing.

I turned my pumps off and drained my bio filter tank in early November. Freezing temperatures make it too risky to run my waterfall during the winter. Once the pumps are off, the bio tank is at risk of cracking from expanding ice.

I use over-sized prefilters for all my pumps. They are a pain to clean, but collect the pond muck before it gets into my pumps and bio filter. Some of my pumps are 12 years old and all are still working fine.

My Savio bio tank contains nylon straps and plastic balls. It provides a surface for bacteria to grow, but the materials don't collect dirt.

Oh, no, Sandy. That is just horrible! I feel so bad for you.

Also, I'm worried to death that this could happen to me too. What causes this? My pond is only 1 1/2 years old. This is my second winter with it and I have 52 small goldfish in it. The pond is about 3500 gallons. I DID NOT CLEAN my Savio biofilter or waterfall filter. By the way this is still running like last winter and the mat filters are still in there. I did NOT vacumm out any debris and there are some leaves on the bottom for the frogs. Will I, too, have this catastrophe?

Sandy,
I ran into this kind of problem once with a new customer (pond maintenance). I couldn't fathom what would cause that level of acidity, I could actually smell it. I did water changes and buffering and then it happened again...
It was vandalism. There was a dispute between the pond owner and a contractor (who built the pond). Failure of payment lead to the vandalism (my best guess would be a gallon of muriatic acid)
Of course nitrites will be high, all beneficial bacteria are dead at that point.

Sandy,
I am sorry for your loss of some of your pets. To me somethings are not making sense. You said that the PH is less then 4 but if this was the case all the pets would be dead so I have to guess that your PH is not that low. But lets say that your PH did crash and you are assuming that the filter caused to crash. And yes a filter can cause a PH crash. This happens because bacteria breaks down ammonia into nitrite and then to nitrate where the hydrogen atoms in the ammonia are released into the water thus lowering the PH. But for this to happen the filter has to be working properly. Did you test for ammonia and what was that reading? Can other things cause a PH crash? Yes the decaying process can also do this by putting CO2 into the water. But I rule that out because you say the pond is very clean and the pond was running. The CO2 build up in the pond would be released into the air and PH should not be affected that much. So what else could have caused? Two other possibilities. The first would be an outside source of an acid. Which is possible but not likely. My guess would be that there algae growing the pond and the pond is covered with shade cloth when the sun is low in the sky because of the time of the year thus keeping the pond in a mostly dark condition. In the dark plants take O2 out of the water and replace it with CO2. At a fast enough rate that the PH will drop. So why would the nitrite be high. If I have venture a guess I would say that the PH got low enough that it killed off the bacteria that convert nitrite to nitrate. This bacteria is easily killed.
Mike

Thanks everyone for the comments. Mike, I believe your probably on the right track concerning the bacteria being killed off by the low PH. Now as for why thats still a unknown. The night I discovered the fish and the problems with them I only had a wide range ph test kit. The same test kit I have been using for several years and the color card shows a range of 5.0 up to 9.0
The color for 5.0 is the color of sammon on the chart and when I took a sample that evening the test tube full of pond water turned to almost pumpkin orange, I had never drawed a tube and tested it with those results and seen anything other then the ideal color that matched a 7.0 -7.5. The color for ideal conditions on the chart shows it should be 7.0 which is a light to med green color. At the time I didn't have a test kit for ammonia and just relied on the PH test that was showing me a pumpkin orange color and at that time I found 4 dead fish and several others suffering. I ran to the computer and looked up what I could do since I didn't have any ph up so I added about 2 cups of backing soda,stress coat and waited about 30 minutes and redone the test and the PH had went up to almost a color that matched between 6.5 and 7.0 on the color chart, which is a light to med green color. The next day I went and got a 5 in 1 test kit and the Nitrite's which should have been 0 with a almost no color on the chart was showing a dark pink color that indicated high Nitrites over the 250 color mark, which I also know is deadly to fish. Right now I justed tested the water again and all the ranges on the 5 in 1 test kit are showing all normal and Ideal levels for the pond as of right now. The 5 in 1 test strips do show different color results then the wide range ph test kit. But I'm sure thats because of the cemicals used to create the test strips are different. But as long as the water test are in the safe or normal range I feel the pond and fish are at least safe for the moment. I'll get the BIO cleaned and set back up maybe this weekend after Christmas. I live way back off the main road on farm land and no close neighbors so vandalism was no way the cause of this. But I think the bacteria for what ever reason did die and the nitrites posioned the water. The BIO filter has been disconnected for several days now and I tested the water in the BIO and the Nitrites are showing a dark pink color on the chart thats is in the range of 250. There is no smell coming from the BIO and the water above the bio ribbon and the plastic scrubbies and the bio mats is clean and very clear. There is yuck brown stuff that to me looks like in any other conditions should be good stuff in a BIO, but I know its not good stuff at the present time.I'll continue testing the water and hope for the best for the remaining fishes.I have drained the pond and right now everything is looking and acting good without the BIO hooked up to the pump. Thanks Sandy

Sandy, I'm worried sick my pond will have the same problems. A landscapers helper blew leaves into the pond and I haven't been able to do anything about it and now I have got to get them out some way but the temps are just too low for me to work outside. There is no ice on the pond right now but now I have some problems from injuries I got when my SUV was wrecked this last Saturday. The injuries are just enough to keep me from using both arms. Very frustrating. Maybe the Prednisone will help.
If the PH continues to be a problem you could try a Plaster of Paris slug. It doesn't depend on the bacteria. It can stay in the water all winter without any harm. Just replace it when it disappears. It will stabilize the PH at about 8.4. (I think that is the number.) That is a good number for the fish. The test results you are getting are typical of the problems when you get with those individual tests. Those and the strip tests seem to give errors all too easily. A new complete test kit bought new every spring does the best job for me. They cost about $25.
From sad experience, I know if the leaves stay in the pond there will be a kill off at some point and I really don't want that to happen again. Also the leaves hide parasites that will attack the fish as soon as the water temps go up.
By the way, the beneficial bacteria stop working and then die off at temps below 50oF. The only filtration that is working for you is mechanical. When temps start to rise in the early spring keep a really close eye out for problems. Sandy

I see no benefit of a bio filter if your pond temperatures are going to remain cold during the winter. The beneficial bacteria will not begin to grow and deal with ammonia until the water temperatures remain above fifty degrees.

The bubbles of an air pump can help bring some of the toxic gas to the surface.

A product I use every winter is called Ammo-Chips. You can find it in the aquarium aisle of most pet stores. Ammonia is produced from decaying organic matter. Leaves, plants and dead fish contribute. Ammo-Chips, and similar products, are made of a mineral called zeolite which naturally absorbs ammonia. I combine zeolite with active carbon pellets in a mesh bag and place it near the bubble stream of my air pump. I don't know of anything else that would reduce ammonia in a frozen pond.

I remove the mesh bag when possible and soak it in aquarium salt, which forces zeolite to release ammonia. Zeolite can be used over and over using this "recharging" process.

I'm not sure if zeolite would work in a pond that contain salt.

Not sure if this will work or not, but if it does this is a little movie clip of my pond early summer of this year. I realize the BIO doesn't do any thing during the winter months, but I do injoy the water fall and it adds oxygen to the water year round. The green net is a shade cloth frame that I place over the pond, but it doesn't not cover up the BIO water fall at all. On cold nights during the winter if the tempature should go down to close to 0 with the wind I cover the whole pond including the BIO tank with a large canves type tarp but I remove it during the day if weather permits. The pond never freezes even with it being mostly above ground. The pond is below ground about waist deep in the middle. Sandy

Well that didn't work, if someone can make it work please do so. Thanks Sandy
[URL=http://s730.photobucket.com/albums/ww310/MissyL_2009/?action=view&current=Picture013.flv][IMG]http://i730.photobucket.com/albums/ww310/MissyL_2009/th_Picture013.jpg[/IMG][/URL]

Sandy,
I am not going to get into weather or not you should run your pond over the winter but depending on where you are in TN I could easily see running the pond as the weather isn't that bad. Somethings that I would like to get into is some of the things that you have said. You live on a farm so it is very likely that you have well water. Being where you are it is very possible that you could have a limestone well. Is your house water very hard? You said that your test was 250 but you don't say what scale this 250 is on so if I assume that it is on ppm or 250 ppm then it is not possible that you were reading nitrite. So I would guess that you actually were reading nitrate. Levels that high of nitrate would harm the fish but not kill them quickly. High levels of nitrite will kill fish quickly and would have killed all the fish quickly. So my next questions are when was the last time you checked on the fish before you found them dead? When was the last time you did a water change on this pond before you found the dead fish? What percent of the pond water was changed at that time?
Zeolite will not work if the pond has salt levels high enough to help the fish in it.
I don't know where this myth that the bacteria in the filter and pond will not work below 50 degrees came from. Yes the bacteria slows down as the temperature gets lower but it will continue to remove ammonia all the way down to 32 degrees. As the water gets colder the less ammonia that the fish put into the pond and the bacteria can easily keep up with this if the water is circulating enough to get the ammonia to the bacteria. The problems usually result from additional ammonia from an outside source such as decaying debris.
Mike

Mike.
Oh no, I just realized how the problem was created this year over pass years. Since I didn't drain and clean the BIO this, which would have been draining out 100 gals and cleaning the BIO, I didn't do a water change other then just adding additional water due to evaporation and what on windy days would get blown from the spill way over flow. Thats got to be why this problem happened. When you ask the question \ When was the last time you did a water change on this pond before you found the dead fish? What percent of the pond water was changed at that time? I realized. Oh my, I didn't change no percent of the water!
Stupid,stupid meeeeeeeeee..With me draining the BOI and cleaning the BIO anually I was keeping everything in check and everthing was fine. I guarantee if I had drained and changed the BIO this year as I had done previous years none of this would have been necessary and my fish would not have died. If I had been doing water changes over the course of the year from the pond maybe the BIO could have handeled not being drained and cleaned. I check the pond daily and found the fish not acting right almost immediately. I'm on city water, not well water, but in the summer i add water by either adding neutralizer or spraying the water from a spray nozzle. I probably didn't have to add more then 40 to 50 gals to the pond all year. I'll definitely not be so stupid in the furture. Thanks Sandy

Mike, Sandy, etc
I like how things get discussed and thought out on this forum.
I'm still trying to get a grip on what would cause such acidity. In really cold water the fish are producing small or no ammonia, decay of any leaves in the pond is also slowed or at a standstill, the beneficial bacteria are there but nearly dormant.
Why the acidity? If the test was faulty (old reagents) it wouldn't explain the fish dieing. I'm also assuming the fish weren't fed in temps below 55 degrees.
My personal experience is that I've seen mismanaged ponds run all year, through the winter in the same zone, full of leaves... and they were fine, healthy ponds.
Once a pond finds its own healthy balance it tends to regulate itself. It may be "neglected" but it is chemically doing great. I have sort of a natural approach to ponding and don't believe in a lot of additives and toying around with the pond, leaving it alone often leads to the best balance.
For instance; problems with excessive ammonia, nitrites and nitrates is usually due to over-stocking and/or over-feeding. The balance gets screwed up and people try to correct that with chemicals.
However, in this case, it sounds like Sandy hasn't been 'toying' with the pond. She was dealing with a strange problem after the fact, in a pond that had been healthy for years...
I know I'm rambling here, just wanted to share htese thoughts.
bubba

Thanks Bubba for the reply. The pond has always been heathly up until now. Here is a couple pictures from earlier in the year. Sandy
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Picture #2
The pond goes below ground level to about waist deep.
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Pond covered with shade cloth Summer 2009
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Sandy, I think you are probably correct about the bio-filter getting dirty and causing the problem.

I shut my bio-filter down in the winter, so a good cleaning can wait until I start it up in the spring. It got down to 18 degrees below zero here last year, so I have major concerns about leaving the falls running.

During warmer weather, the good bacteria turns ammonia into nitrites that oxidizes into nitrates. Nitrates are used in commercial fertilizer and your pond plants will absorb them. Nitrates can also kill fish and if your plants are dormant during winter, they will remain in your pond.

I would also be careful with winter water changes using "city water". A lot of drinking water is treated with monochloramine which is formed when ammonia is added to chlorine. When you add a dechlorinator to your water it removes the chlorine but leaves behind the ammonia. Without bacteria, the ammonia can quickly become a problem.

I've linked some bio-filter information posted by a well respected member of this forum. I hope DRH1 doesn't mind.

Here is a link that might be useful: When do I start/stop my bio-filter?

Sandy,
Those are nice pics. The first one says it all; well built, well maintained, not over-stocked. If anything the filter might be too big :)
Bubba

Bubba I would guess that the PH drop was caused by the shade cloth keeping the pond in the dark causing the algae to take O2 out of the water and create CO2 which caused the PH to drop. I would assume that the pond water has a low alkalinity so nothing was there to stop the PH from going down. I would guess that you may have the same situation in GA. You said that "In really cold water the fish are producing small or no ammonia, decay of any leaves in the pond is also slowed or at a standstill, the beneficial bacteria are there but nearly dormant." Everything in this statement is true except the part about the decay of the leaves. A couple of years ago I did an experiment where I left the leaves in a 4500 gallon pond and left it dormant all winter in IL. It had temperature probes at varies places in the pond. The pond did have about a 1.5" layer of leaves on the bottom. With at least 8" of ice on the top of the pond the temp in the leaves stayed in the 70's for most of the winter. The only way that the temp could be that high is for a lot of decay going on. The temp just above the leaves stayed 38 degrees. To make a long story short the normal alkalinity of this pond was 450 ppm with a PH of 8.3. Mid Jan the PH was down under 7 and the CO2 over 30 ppm. All the results were posted here a couple of years ago.

John the thing that you forget is that Sandy is quite a bit south of you , me, or DRH1 and what we would do doesn't apply to her. One of my ponds is covered with a greenhouse which is about the same as that pond being about where Sandy is. The pond never drops below 40 degrees with only a little ice ever forming around the edge. I have no problem leaving everything running.

There is no such thing as the filter being too large for a pond.
Mike

Mike, no offenses but you need to start reading posts before offering solutions. Sandy stated in her post that the problem started when the temperature dropped to 17 degrees. At that temperature the bacteria in her Savio bio-filter stopped working and the buildup of pond waste most likely created ammonia which resulted in her loss of fish. I think Sandy came to this conclusion herself.

John, I don't take offense at someone questioning my solutions. What it means was that my solution was either wrong or I was unclear with my solution. The problem with looking at just one piece of the puzzle is you don't see the whole picture. So lets look at the one piece that you looked at,17 degrees. 17 degrees is the air temperature and not the water temperature. Sandy never said what the water temperature was but I think it is safe to say that it was over 32 degrees. The bacteria in her filter never stopped working. This can be seen by the fact that one test reading she gave besides PH was 250 nitrite. Since we both know that 250 nitrite can't be right as nothing would be alive at that level. So it would have to be either .25 nitrite or 250 nitrate. Either number is the result of bacteria do something. I am not saying that you are wrong about ammonia killing the fish but I think you maybe wrong. Sandy checks the fish once a day. It would take a very high level of ammonia to kill fish in one day which most likely would likely kill them all. I do not know what Sandy thought killed her fish other than there was no water change. If the one reading is 250 ppm nitrate than that would put the fish under stress. Add to that the low PH and some of the fish would start to die and the rest would be in danger of dying. The water change that she did would have raised the PH and lowered the nitrate level and the living fish would recover quickly. Yes a water change would have lowered the ammonia level temporarily but if the debris was the cause it would have just gone back up and the fish would start dying again quickly. Are Sandy's fish out of danger? No because what caused the PH to drop has not been corrected. Yes it has been slowed down with baking soda but not corrected.
Now John I won't say that I may not have caught everything in every post but I have read every post. What I have learned in looking at hundreds of fish losses is that it usually isn't just one thing.
Mike

Didn't really wont to bring this post back up to the top but i thought I would pass along how things have been going and since the pictures of my pond where already here I brought it back to the top . Well one word would explain a lot, that word would be COLD here in middle TN, at least colder then what we are costume to, but nothing to whatsome of you have been having to deal with. My pond being mostly above ground and in the middle of my yard has faired rather well considing everything eles frozen solid around here. I had no choice but to pertect the pond from the cold in what ever way I could find. See pictures above of my above ground pond. The coldest day was during the night this past Saturday with a morning temp Sunday morning of 4 degress, with windchill of -10 Burrrrrrrrrr.. The cold weather moved in the first of last week, snowed some, but the temps and the windchill was what had me veryyyyyyy worried. I purchased a duct brown canvus type tarp that would fit over the entire pond all the way down to the ground and used stretch cords to secure the tarp down around the rail road ties and had the pond pump about a foot below the water surface for water movement. I had already taken down the stock tank bio filter due to the fish death's I had early in December. Well when Saturday came and the weather forcast was giving out near 0, I went out and put another heavy duty tarp over the pond and stretch cords around it again. I tested the water before I put on the first tarp and everything looked good, but I was worried since the pond has been covered now for about 6 days yesterday, I could here the water moving due to the pump so I knew it wasn't frozen. I went out and undone one corner and tested the water and all the readings looked purfect and I turned off the pump for a few minutes and had to use a flashlight to see if the fishies where alive and doing ok..They where just fine and seem to have survived this cold spell without any problems. The tarps saved my little pond form a total disaster I sure. To my surprise there wasn't any ice that had formed in the pond during the last 6 days. I had night mares dreaming of my pond freezing solid and expanding and busting at the seems pushing out the rail road ties and being nothing but a big block of ice. I counted 13 kio, and goldfish and no one was floating. So maybe I can relax at least until the next round of old winter blowes in. Sandy

Sandy,
Glad to hear everyone came through ok this time. I'm sure covering your pond kept it cozy and warm.
Don't worry about the fish being in the dark for a few days - up here in the frozen north, my fish have been in the dark, under the ice and snow for a month. You'd think they'd like to be under the cover where it gets warm and sunny but no, they swim 'round and 'round in the deep end under the ice in the dark.