Question
I live in Phx.  I have a pond (2 ft. deep) which recieves sun from approx. 9am to 3pm.  About a month ago I put in 3 feeder goldfish and a small koi. Within a week and a half, all that was left was the largest of the feeder goldfish.  Since that time, it flourished.  I would only feed it a few pellets every few days.  There are plants, algea, rocks, etc. in the pond so I figured it was doing well. It grew twice the size from when we got him.  A few weeks ago, the water was murky.  I drained it down 3/4 the way, hosed off the rocks on the upper plant shelves (while it was draining) and refilled.  The fish did fine.  No issues.  Repeated this process yesterday.  While the upper levels were drained, I noticed my plants were growing out of their containers. ( 2 canna plants, a miniature cattail, and a pickerel plant)  I created a wall with stone, lined the inside with pond liner, then filled the cavities with dirt and placed the plants directly into the dirt.  Refilled.
Very soon after the pond reached it's full level, the goldfish came to the surface.  He spent the remaining 30 minutes of his life "gasping for air".  My first thought was I had depleted too much oxygenated water, even though I left the waterfall running for the last 10 minutes of drainage.  I also left the same amount of water in the pond as I did last time, but wondered if the heat we're in now (as opposed to earlier) depleted some of the O2 as well.  
I have read that if the pH is too high, it can cause that sympton of "gasping" at the surface.  AZ soil is a heavy clay, and very alkalinic.  Do you think having that much dirt exposed to the water threw off the pH? I plan on testing it tonight when I get home.

Very disappointed, as that fish looked healthy and was growing like a weed.  

Answer
I'm sorry about your fish.  How did the fish that died earlier die?  As for this last guy, there are a number of possibilities.  First, a 75% water change is very stressful because of the following - changes in water chemistry, reduction in oxygen levels, chlorine/chloramine if you have city water, and so on.  I suggest testing your pond's pH, hardness, ammonia, nitrite, and oxygen and chlorine if possible (those last two test kits are harder to get).  Is the pH of your tap water different than what's in the pond?  What about the hardness? Did you add dechlorinator (if you have city water) with the water change?  Tap water is low in oxygen.  See my page at http://www.fishpondinfo.com/fishcare/water.htm for more information on why.  So, the first issue is the water itself and the water change.  Then, there is the dirt.  Normally, when you pot plants, you want to top off the dirt with a few inches of pea gravel to hold in the dirt so the water does not cloud.  Most dirt should be pretty safe for the fish but it's possible it was too alkaline or even contained chemicals such as pesticides/herbicides (unless it was from your land, and you know that not to be possible).  Dirt itself can also clog fish gills to some extent making it harder to breathe although that alone should not be fatal.  It sounds like your poor goldfish suffocated.  That can be due just to the large water change or the presence of poisons which include chlorine, ammonia, large pH shifts (acids or bases), etc.  The poisons make it harder for the gills to do their job.  When a fish dies from high ammonia or chlorine, they gasp.  It could also be that the combination of the large water change with its change in water chemistry and lowering of oxygen on top of something coming from the dirt was just too much.  

When you say your water was murky, was it white, green, brown?  That's a separate issue that I may be able to help you with.  It's better to deal with the cause instead of just doing large stressful water changes.

So, I suggest you test what you can in the water, put pea gravel over the dirt, filter and aerate the water well, and maybe try some more small fish.  I hope things improve!

Robyn
fishpondinfo.com