Either way also can, egg or coin or any other common object which has no sharp edge. Putting egg in is just a common practice in Japan. We can put other thing in too... like erm... ... spoon, etc.

It is just to give indication of the size. It is better to have something similar to the fish size. Now, coin is a little small comparing to the fish. (Just imagine how fast they grow)

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Japanese Ranchu is another ball game