man, so many things wrong in this situation. Don't mean to offend, but to put 20 koi in a 100 gallon tank is not a wise choice. Its not impossible but it means daily water changes and a MUCH MUCH bigger filter then that little Home Depot Becket thing.

The reason for their death could be so many different things. Probably bacterial infections from being so overstocked and under filtered. But it could be ammonia, could be parasites, could be toxins from rain run off. Could have been sick right from the start. could be a low kH causing the pH to crash.

When keeping heavy stock in small water, problems become lethal very quickly. You can lose them all in a matter of hours if you are not careful.

first things first. Start doing daily water changes of about 25%...don't forget to dechlor. I suggest Amquel or chloram-X because the dechlor and remove ammonia at the same time.

next buy yourself a good test kit. not those little strips, but a real test kit. it should test for ammonia, nitrites, nitrates, pH, and kH. A salt test would be nice too. I suggest the Aquarium Pharamcuticals freshwater pro test kit.

lastly, prevent run off from the roof in the future.

once you get the kit, post the results so we can take a look and maybe figure out what is wrong.

Thanks, no offense taken. The pond pump/filter is supposed to be for a seven hundred gallon pond. Mine if filled to capacity is only one hundred, but is usually ten inches below top level.. The fish are only four inches long. Is there any ration of fish/fish size or how many at what size, per gallon??
I'll get the tester. I'll drain and refill the container each weekend. I've been using de-chlor with each refill. I'll protect the container from roof run off. It had never occured to me to do so, until I saw dead fish and roof rain running into the container.
Thanks, I am new to this. I do want to do this correctly.

Usually it's 1 inch of fish per 2 gallons of water. And remember Koi grow!! So you need to take into account their changing size. You have twenty fish. Lets take an averge of 3 inches per fish. That's a minimun of a 120 gallon tank. In a month they will be bigger and you'll need a bigger tank.
Surfhead has a good idea. Start with daily water changes and get a test kit. Lets us know how it goes!

There is no real ratio per inch. On average its more like 10 gallons per inch, not 2. But really it all depends on the size of the filter. You can have 100 fish in a 100 gallon tank but you need a filter to handle the bio load of 100 fish, not 100 gallons. Those little becket filters say they can filter a 750 gallon pond, but that is assuming a VERY low stocking density. Maybe 2 or 3 fish. Its all a ratio of the amount of waste the pond has vs the amount of bio filtration to convert that waste. The number of gallons is almost irrelevant when it comes to the nitrification cycle.

It's not just the number of fish, size of fish and size of filtration, but also the amount food and the quality of food. High amounts of food mean more waste. and cheap food has fillers which means more waste as well. So when heavily stocked feed lightly with a high quality food.

Ok, all but eight fish perished. I've emptied and cleaned the pond. Fresh dechlorinated water. Added a much larger pump. I want to look into the possibillity of a better filter system as well. Mine is the slightly bigger than a shoebox HomeDepot bio system with dual mesh materials and bio balls. No uv. Suggestions of an affordable system will be appreciated. Remember, i'm a hobbiest, not a commercial grower. :)
Moved the pond to where no roof rain spill off will affect it.
I'm treating what I believe is SAP fungus on a few of the remaining fish. This will take a few days to a week if I understand correctly. The fungus supposedly thrives in cooler temps. My weather has been that unfortunately.
Purchased the 'master water testing kit'. Tested three times this weekend. In the newly setup tub, most everything reads ok. Ammonia, nitrites, nitrates, everything except the Ph. It was on the high end of alkalinity. Might be the added salt? Anyways, I added 'Ph down' and by the third test, it had been reduced somewhat. I'll keep my eye on it, and continue testing.
I won't add more fish until I either get a better handle on all of this or a larger tub.
I thank who ever turned me onto Koiphen. Very informative. I believe i'm on the right track. Continued suggestions are more than welcome.

glad to see you have gone to koiphen...lots of good info there.

please do NOT add ph down. a high pH is FINE. the most important thing is a STABLE pH. You want a high alkalinity (kH), this will stabalize the pH. When you start adding pH up or down then you start this rollercoaster with the pH and that is the worst thing you can do. Just keep the kH high and all will be well. salt will not effect the pH or the kH.

If you want a better filter for less money, go DIY. I like to use empty 55 gallon barrels. Car washes will usually let you have them for free. Make sure you rinse REALLY well and use Potassium Permagante to steralize and remove all toxins. Then I like to fill with strapping material. You can get 900 feet of it for around $40. Its just back plastic poly strapping....makes GREAT bio media. The pump pulls the water up to the top and you rig a PVC spray bar to spray the water down over the media. Then near the bottom you glue in a 2" shower drain ($5 at Home Depot) and run 2" ABS pipe back to the tub. For less then $100 and a weekend you can have a really effective bio filter. You can keep your beckett as a prefilter.