fish kill in 700 gallon pond... help!

[RinsMom]

We just noticed about 10 dead fish in our goldfish pond...  all goldfish (commons and comets, probably).  I am currently testing the water, but it appears that the nitrates are 0.  The pond is heavily planted with water lilies, for one thing.  (Nitrates are zero.  I didn't test for anything else, but I probably will.)  It has been cycled for many years.

Some background info. ... the pond has been set up for about 8 years, and is very self-sustaining.  Every other time I test the water, it all comes up as 0, 0, and 0 (ammonia, nitrites and nitrates).

There is usually a population of 30 to 40 goldfish, and 1 koi. We also have a lively crop of frogs in the pond. The fish range in size from fry to a 6" goldfish.  I don't really feed them very often, and when I do, I feed them a sinking pellet on a submerged platform (we had problems with predation when I surface fed).

Additional information.. I am in Central Texas (just north of Austin), and the last few days have been in the upper 90's during the day, and 75-80 at night.  The pond is mostly above ground (about 2 feet deep), with the back portion submerged (it is on a slope).  Yes, during the summer the water does get quite warm, but the area that is buried is cooler, and 2 rock ledges provide nice shade and cooler water.  A pump circulates the water through a "lava rock" trickle tower above the pond.

We have not used any chemicals in the yard.. we don't use chemicals in our yard, and we have 1/2 acre, so it is unlikely that toxins drifted over from  a neighbor.

I do not see any signs of injury or obvious illness... some were bloated (and extremely smelly) as they may have been dead for a couple of days.  I've pulled out about 10 dead fish, ranging in size from 1" to 3".

Is there any easy way to test for dissolved oxygen? I am wondering about excess plant decay and build-up on the bottom of the pond, creating an oxygen depletion.

Any suggestions?

[Fluffy92]

What is the temperature of the pond?
Is it a possible pesticide problem? Have you used any? Your neighbours?

Is any other death such as frogs? They are great indicators of posioning because they breathe through their skin so if its the water quality, you be seeing dead frogs. If you not then its likely something to do with only gold fish.

[RinsMom]

My son saw several live frogs in the pond when he went out to check it. I didn't see any dead frogs, but I don't know the status of any tadpoles..  I'm not sure when (month wise) we usually have tadpoles.  We've not used any chemicals of any kind, and there is a privacy fence between us and the closest neighbor.  We heard the frogs and toads singing just the other night.. we have a large colony of Gulf Coast Toads in our yard.

I don't know what the temperature of the water was...  it does get real warm, and I realize the goldfish do better in cooler water.  When it gets hot like this, they go for the areas shaded by the rocks..you can feel a definate cooler area under the rocks.

We are in the middle of a storm, so that is cooling off the water tremendously.  However, the temp of the water near the surface probably stays at 80-85 degrees during the summer, with it being probably around 75 degrees in the cooler areas.

[Cholly]

I wonder if it might not have been a cascade effect, higher temp, lower O2 content, weakest fish died, causing ammonia level to spike making it harder to respire, next weakest died and so on until the O2 in the water was better able to support the remaining number of fish and the bacterial cycle caught up.
No new fish had been added had they?
I'm racking my brain, but that's all the possibles I can come up with at the moment. I'll let it percolate a bit and see if something else occurs to me.

[Fluffy92]

hmm if there are no frog death then the water is pefectly fine. You can rule out problems with the pond chemistry. It would be a oxygen problem in the water. Has frogs and other wet skin amphibians been leaving the pond? Instead of in water they are out of it, on leaves, ground, dirt? That would also mean bad water chemistry.

Does your pond have a lot of dead plant matter? Algae? Anyway brown sluggish guck at the bottom? On hot days, plants tend to decay faster then usual and they absorb too much oxygen and release far too much carbon dioxide.

If all of the above does not exist then I think you might be dealing with something solely attacking the goldish.

[RinsMom]

There is a lot of muck in the bottom of the pond.. all of the plants have escaped their pots, and the root masses alone are probably an inch thick.

I do try to keep the dead lily leaves out of the water, but as crazy as it has been around here these last few weeks (final planning for my daughter's wedding which was yesterday), that may have been a lot of it.

I have a feeling it was primarily the oxygen problem, from the muck, and then the extreme heat, then as Cholly suggested, a cascade from an ammonia spike from the decaying fish.

I haven't really checked it since that initial discovery, (other than a quick scan and disturbed the lily leaves to see if any more dead fish were hiding), but things look pretty good.  I'll try to do a big water change today if I can.. about 100 gallons or so.  That ought to help.  At the same time, if I can reach some of it, I'll try to suck up some of the muck if possible.

[RinsMom]

I was just outside checking the pond, and things look pretty good.. I found another dead fish (but it may have been hidden under the water lily leaves), but there are also a BUNCH of tadpoles.  I have a feeling that it was an oxygen problem.  I'll do a big water change as planned today and suck out as much of the muck as I can.

Thanks, folks!

[anshuman]

After all this months, how is the issue now? did you add new fish and are they doing good?

[RinsMom]

I didn't add any fish....  I may wait until spring.  The way our summers have been going, it always gets very hot.  Without the pond being insulated by soil (it is above ground), this may be a continual issue.  Even with the heavy lily cover, the water gets quite warm near the top.  (It is cooler at the bottom, of course..)

All summer, we probably had 5-6 different frogs in the pond, with the resulting tadpoles... I have a feeling it was mostly a low oxygen problem.

There are probably two dozen or so goldfish still in there..... and they will help re-populate the pond next spring and summer, also.