Hard to believe it would be parasites with such sudden deaths. Even though you have a seasoned pond I would still do a full water parameter test. It is possible if your pond is not buffered sufficiently to have a PH crash which would kill all your fish real quick. Some ponders depending on their local water source have to keep crushed oyster shells or add baking soda on a regular basis especially if they do regular water changes. That's where I would start.

frogman4: Thank you for the suggestion. A pH crash didn't occur to me since I have hard water. I should have run water tests right away. Here are the results for the test parameters I have kits for.

pH: 7.8
Ammonia: 0 ppm
Nitrite: 0 ppm
Nitrate: 5 ppm

Maybe this is a silly question, but could the pH crash and quickly rebound?

I was up until 2 A.M. reading about possible causes and am still at a total blank. I'm wracking my brain trying to retrace my steps to determine if I did anything different or if any substance could have come into contact with it.

For what it's worth, my deceased fish had good color, nice fins (no signs of tears or tattered edges) and their eyes were clear (not bulging, cloudy, etc.).

I'm in the process now of draining it. I will scrub the algae off the sides and the rocks of the waterfall, rinse, scrub and rinse again. I'm not sure what else to do to clean it. Any suggestions would be much appreciated.

Anne: Thank you for the kind words. Right now, it's depressing to even look outside at my pond. I'm also nervous about reintroducing new fish once it's drained and restarted. I don't want this to happen again.

Sorry to hear about your loss ck....Did it get really hot out, so that maybe the water warmed up more than usual? I think I remember reading once that warm water will spike the ammonia level.

If I had to guess I would say you had either a nitrite spike or poison got into the pond.
Mike

With perfect water parameters and healthy fish the day before, the only thing I can think of is poisoning, whether deliberate or accidental. You might talk to your neighbors, telling them that your fish died of apparent poisoning and ask them if they saw any strangers around your house or observed anyone spraying anything in the area. You may get an answer if it was an accident. If some kid thought pouring something in your pond was a cute idea, the message will certainly get around that everyone is looking for the culprit.

Since I used to work in a Radio Shack many years ago, a fairly common problem occurred to me. A small electrical leak can kill fish and a lightning strike might, too. It would explain why all the fish were affected. The leak is called stray voltage and often affects farms where wiring is sometimes not up to code or becomes worn.

When we were staying in a townhouse complex while looking for a house, I discovered an electrical leak by walking on damp grass in my bare feet. It was just a tickle but it definitely was live electricity. The entire yard had to be dug up and they discovered the wiring coating had been chewed by critters. No conduit, wrong type of wire! The only reason I noticed was because the grass was wet and so the charge was greater.

Usually a fish killed by electricity(or a lightning strike) will have a definite bend in it's spine.

I am sorry about your fish. It has to be something like electricity, low oxygen (they were fine until they got too big for the gallons of the pond) or poison. Not disease or parasites. It is not unheard of for evil people to dump bleach or other chemicals in ponds....

Scrubbing all the algae off the rocks and walls is going to feed an algae bloom when you fill it back up. Carpet algae is good algae.

Hi everyone,

Thank you all for chiming in with possible causes to look into further. Right now, I'm leaning toward something getting into the water. If it is a poison, what should I be doing to make sure the pond is safe for future fish? Over the weekend, I scrubbed it down with a stiff brush and rinsed with water. Is this enough?

bonniez3: Thank you for the kind words of support. The weather has been nice here. The temperatures have started rising, but there haven't been any drastic spikes.

Mike: I've read up about nitrite poisoning and am leaning toward the idea some poison or toxin got in.

Sharon: That's a great idea, but my neighborhood isn't the most sociable. There are no disputes or anything of that nature, but outside of saying 'hello' when getting the mail, we rarely converse.

sleepless: An electrical short is an intriguing possibility. There are two in pond lights, and one keeps going out despite replacing the bulb and checking the connection. It is low voltage, and I didn't feel any type of shock or tickle when I put my hand in the water. Also, there were no discernable bends in the spines of the fish, but I will certainly replace that light fixture!

buyorsell: If a poison is the cause of my fish deaths, I am afraid it may still be present in the algae, hence the scrubbing. I'd much rather deal with an algae bloom than risk future fish dying because I didn't clean the pond thoroughly.

Once again, many thanks to everyone for your advice! :-)

CKR97,
You don't happen to have an ion generator on the pond? That could easily kill all the fish in the pond if the PH dropped below 7 for even a little while. The reason I mentioned nitrite poisoning is that when happens it will kill all the fish fast. Then the levels can drop back to normal. But I would say that isn't very likely. Poison is the most likely. Depending on the source and type of poison it maybe gone or still there. Usually with most poisons all you have to do is empty and refill the pond and you reduced the amount of poison to a point where it is not a problem.
Mike

I would definitely get rid of that light. The only way you might determine if it is at fault would be to check it with a multi meter. Even then it could be that the light is reacting to a surge in the power levels. Do you sometimes get brownouts or power failures? Is the outlet grounded? Often older wiring was not properly grounded because the devices we used before the electronics age could handle uneven power input.

Our house is not all that old but we do have problems with the wiring. There isn't enough of it for a modern lifestyle to start with and most of the sockets have reversed connections. All of the power supplies for computers and electronic devices have to be plugged in upside down. Other things require an adapter. The only line that isn't a problem is the one we use for the pond. Lightbulbs burn out at a horrible rate and I can't run a hairdryer.

You may not have felt a charge because you are a lot bigger than the fish and you are on dry land and grounded.

We were getting ready for my sons wedding.It was to be on gazebo over hanging my pond.We have swans,pond was loaded with fish.A few days lijke 2 days before weddinhg I drove in driveway ,pond is right there.I saw something over the whole top of pond.I went down.Oh my gosh dead fish everywhere,hundreds.20" bass.14"cat fish,dozens blue gills.Sick,sick.Wcalled someone specialist in this and was told this is natures way of cleaning out.Depleation of oxygen.A friend of ours had the same thing.

Mike: I do not have an ion generator. It's reassuring to know that draining and refilling the pond should be enough to dilute any poisons. It appears I was rather extreme to scrub all the algae off, but I do feel better about safely adding new fish.

sleepless: I can't recall the last time there was a power outage. An electrician ran the wires to the pond and they are properly grounded. The outlets are all GFCI. My pond is on its own circuit breaker separate from other lines in the yard (swimming pool, decorative lighting, etc.).

As for the in-pond fixtures, the bulbs have always lasted for years. I probably only replace them every 2 years or so. When this one fixture burnt out about 6 weeks ago, I replaced the bulb. It went out again about 3 weeks later. When I opened the fixture up to replace the bulb again, it was dry with no water leaking in. There was no sign of any damage to the wires, so I chalked it up to a bad bulb. I will certainly toss it and get a new one. There are some great prices online for new fixtures, so it isn't worth the risk to keep it, IMO.

bulldinkie: That's terrible what happened to your pond! Not only did you suffer the massive loss of your fish, but it happened so close to a special day for your family. I hope your pond has recovered and is doing well now.

Oxygen depletion kills the biggest fish first and it is very fast. Usually happens at night or very early in the morning.

Go out and check your fish very early in the morning. If they are gasping frantically at the surface of the water, you've got a problem. Levels can rise during the day and you can never know until the biggest start dying.

If you have ever seen them gasp like this you won't mistake it for normal behavior. It is desperate.

Hi buyorsell:

When I first started researching what could have happened in my pond, every article online pointed foremost to oxygen depletion. But I find that difficult to accept as the cause of my fish deaths because I have ample water movement. There is a large waterfall which runs 24/7 and there are no still or stagnant areas in my pond.

In addition, there is further aeration provided by a small pump with a water feature attachment.

When it was first up and running, I worried that there was too much movement, yet it never seemed to bother the fish. Being in the Arizona desert, temperatures get up to 115 degrees in the summer and I've never had any issues with the water getting too warm.

When I'd head outside to watch my fish, I'd have to sneak up and keep out of sight. Sometimes they would be lazy and causally swim about, other times they would playfully chase one another. No matter what they were doing, as soon as they spotted me, they'd dart over to the location where I fed them and stick their heads out of the water begging for food. :-) Gluttons. :-)

They may have grown too large for the oxygen in the pond to support them. Were they all the same size/age?

You may never know the exact cause. I know it is frustrating and upsetting. I've lost fishies too. One winter three years ago some bird that I never saw took over twenty without a trace. Some were twelve years old.

I grew up in Phoenix, BTW, had my first pond there. Am headed home in October for my 30th Reunion.

My poor dead koi died last week from oxygen depletion but that was from a power outage. They were dead so quickly I never even saw then gasp because it happened at night. After we got the generator it saved the rest of the fish but my tamest and oldest and biggest are all dead.I'd suspect poison in your case. Sorry for your loss. I miss my guys.

My sister-in- law grabbed a can of plant food and sprinkled it in her pond without ever realizing what she did until all her fish died. The only reason she could figure it out was because she put it away where she kept the fish food and nothing else. She cried for months.

I knew someone years ago who had neighborhood teenagers dump gallons of bleach into her pond while she was on vacation.....

Dear ckr97,

I realize you'd posted quite a few years ago, and that you've probably found out what killed your fish. I hope your pond is now repopulated, and doing well. Recently, I had something similar happen to my goldfish that I'd had for years in a large pool I have out deep in the woods. In March 2015, I spot checked, and found 5 of my goldfish floating dead, leaving 3 of the big ones alive, and all of the little ones (7) dead. When I put my hand in the water, I felt a 'tingle' of electricity. Upon investigation, I found that the pond had flooded (we had a lot of rain in a short period of time) and the connections to the pumps had ended up being under water. I checked with the electric company, and sure enough, my electric (which comes from an RV pole I have out by the pool, just for the purpose of the fountains) bill had spiked considerably during this time frame. Just wanted to share my story, in the hopes that someone else's beloved fish don't end up electrocuted like my sweeties.

@Pavolizard, you really need to have your pond pumps and such connected to a GFI switch. You're quite lucky you didn't kill yourself when you put your hand in the water.