The sad reality is that goldfish produce more eggs than can be imagined and certainly more fry than can be raised to maturity under any reasonable scenario. In addition, if you are breeding fish to a certain type, usually only a very small number will be as good as their parents (or even true to type sometimes). In order to raise goldfish successfully with finite resources, the hobbyist has to continually reduce the population to a manageable level.
The first opportunity to do this culling is at 10 days to 2 weeks. My fry were about 3/8 of an inch long and recognizable as little fish. I was raising Ranchu, which have double tails. At 2 weeks I removed all single-tailed fish and those with really obvious tail deformities. This took me about 24 hours (in 2 sittings) to do and removed about 50% of the fry. I took the culled fry to a local pet shop to use as fish food and sell to people with ponds (as mosquito eaters? I don’t know). Unless you have very good stock, I would not expect to get money for your fish until the 3rd or 4th culling when they are several inches long.
To do the selection, I put the fry into 2-gallon buckets. I then put scoops of about 30 fry at a time into a shallow porcelain dish. I used a baster to pull out the deformed fry, and then poured the remaining good fry into a new bucket. The important point is that the fry are quite delicate at this point: you want to avoid directly handling the good ones other than scooping up in a cup and gently pouring into buckets.
Above: Some tools I used for culling the fry. Note the 2 week old fry in the porcelain dish. I have heard that the traditional tiny, flat net is much easier to use than my "baster" method.
I was fairly conservative in culling: if in doubt, I kept the fish. For such a large spawn this was probably not too good an idea. I spoke to an experienced breeder who suggested just getting rid of all but a few hundred fry. The idea is that I would have been better off with a smaller population that I could take better care of than a huge spawn that I would always be struggling with. Advice is easy to give: if in doubt cull the fish.
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