50% of koi have died within a 12 hr. period

Question
My pond is above ground and approximatley 200 gallons. All of the koi have been in this pond for 4 plus years and not a single incident the entire time. I noticed most of the koi were on the bottom and not moving at all.Then 1 by 1 they staryted dying., 5 total so far. What the #*&%$# is going on? Help!

Answer
It is extremely difficult to diagnose a problem like this when there are no visible signs of disease on the fish.
The very first thing to do is check ammonia and nitrite levels of the pond water.
If those turn out okay, then you must assume that some sort of poison got into the water (maybe the neighbor sprayed his fruit trees or lawn fertilizer washed into it during a rain).
In either case immediate first aid must be administered. Change 1/2 the water at once. Use a good de-chlorinator, preferably one that helps restore the slime coat. Add pond salt at the rate of 1 1/2 cup per 200 gallons.
If the water temperature has started to cool in your area then the water change must be done very carefully and slowly - Koi are extremely sensitive to sudden temperature changes and the water in your garden hose may be a different temperature than in your pond.
Repeat this process several times until the fish are acting normal again.
If the problem is because of nitrite/ammonia levels then the partial water changes are still necessary, but feeding must be stopped until those levels return to 0. Additions of beneficial bacteria will help alleviate the problem and good products are available to remove/reduce toxidity of those poisons (PondMarket Koi TLC and Pond Care Stress Coat come to mind).