copper sulphate (Crystal Blue) product killed my Koi. What now

A. You picked about the fastest and surest fish poison that exists, to put into your tank. Drain, flush, drain, flush, and so on until copper concentration is down to parts per billion, and then fish will not be poisoned.

A. If the copper concentration killed the first fish, why would you not expect it to kill the fish each time afterwards?
It strongly appears that you either did not weigh properly or you miscalculated the total gallons of water.

Drain it and flush it several times and start over.

A. I had a similar occurrence with my fresh water tropical fish. One of the dead fish was at least 15 years old! I also have a pond in my garden that is very prone to algae and I find the best natural treatment is to use barley hay. Stuff a pair of your wife's tights with the barley hay and throw it into the water (preferably without her in the tights!). After a few days the hay will start to sink and as it does, it clears the pond water. I would suggest you tie a chord to the tights so you can get them out again. Leave it in the pond for the whole season and replace it as needed. Over here they are now selling concentrated barley hay extract, but I have never used it. I prefer the natural stuff as it offers nutrients to the plants as well as killing the algae. Be warned, you must use barley hay, nothing else works. I would also suggest it is one that is not treated with any artificial chemicals such as pesticides.