First off, I doubt copper accumulates in goldfish. Copper sulfate is used to treat food fish and the FDA gets a little concerned about things that can accumulate. Now malachite green can be used on "bait fish" and it can be absorbed by fish. This substance is an organic coal tar dye, whose color resembles the color of copper mineral malachite but it does not contain copper. It also is a nasty chemical but I don't think it's used routinely in gold fish production. Common salt would be the most likely treatment.
So what is the problem with goldfish? It is something that is natural in them. This is the enzyme thiaminase, this enzyme breaks down the vitamin B-1. This most likely the culprit in deaths of animals fed goldfish as a sole food. Fish in the minnow family share this enzyme, ie rosy reds, smelt.
So what to do the obvious simple thing is to gut load the gold fish with food high in B-1. The other would be to use non-goldfish. I found this list on a website see the link: http://www.austinsturtlepage.com/Articles/Thiaminase.htm
Note Sunfish do not contain this enzyme and would make a more natural diet. As for as crayfish go you might check seafood stores for live crayfish. Doing a web search for live louisiana crayfish turns up a lot of people that ship live crayfish from bait to super jumbo size. 5lbs of crawdads should last you awhile. Sachs Aquaculture sells small quanities of cray fish see: http://aquaculturestore.com/index.html

HTH
Thiaminase

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