Question
last week the pond received and excessive amount of run off from culverts to yards back to the pond and the water turned dark then the fish started dieing, first the bait fish then the carp and bass then the cats but the tilapia and perch are surviving two days afterwards the oxygen level read 2.02 mg/l what has happened to the pond? what has made the oxygen level drop? why did some species live? and when the water changed to the dark color did that effect anything?

Answer
A lot depends on what was in the runoff.  First, there was dirt and silt.  With that came any pesticides, herbicides, oils, etc. that may have run into the culverts.  If there were poisons in there, those alone would harm the fish.  The dark color could be from various dirt/soil or perhaps chemicals.  Rain water is low in pH and oxygen.  That also would lower those things in the pond.  So, your pond was hit with a number of problems all at once leading to an oxygen too low to sustain the more sensitive fish.  This excerpt is from one of my newsletters:

"I had told my boss about my fish dying and my thinking it was oxygen related. He told me to bring in water for oxygen testing which I did on 7/17/06. I thought they weren't going to do it but another chemist gave me the results on 7/25/06. My boss claims they are valid even though the sample was in the refrigerator for over a week. The dissolved oxygen (DO) in their aerated distilled water was 8.7 ppm (near saturation). The DO in my pond water sample was 5.7 ppm. In looking for the ideal concentration, each web site says something different. Basically, below 4 ppm is bad but fish may not die until below 1 ppm. A reading of 5-6 ppm is good according to some pages but others say that coldwater fish need at least 6 ppm and koi 8 ppm which is near saturation. So, while my pond water is lower in oxygen than I would like, it shouldn't have been deadly. The sample was taken in the early morning when oxygen levels are the lowest. I haven't lost any more fish since 7/12/06. I also read the pH and got 6.25 on the meter which seems way too low. The meter wasn't stable. I intended to test my pond water at home that night."

So, at 2.02 mg/L (the same as ppm), your water was deadly.  Some fish can live with less oxygen or gulp some atmospheric oxygen.  I guess the tilapia and perch are more able to cope with lower oxygen levels.

I hope you can get an aerator if you don't have one.  See my farm pond page at http://www.fishpondinfo.com/farm.htm for links to stores that cater to farm and game fish ponds.  They sell various aeration devices to help your fish.  If the dark color is simply due to natural dirt and such, it should settle out and not cause further harm.  If, however, toxins got into the pond from other people's yards, then they may still be causing problems.  How big is your pond?  For smaller ponds, I would suggest activated carbon but for big ponds, you would need so much that you couldn't afford it.  The places I mentioned that I link to on my farm pond page can probably give you some advice in that area since I know more about smaller ponds than larger ones.  Hopefully with aeration, your pond will recover.  Good luck!

Robyn