Those feeder goldfishes are culls from fancy goldfish spawns, ie they didn't quite make the standard. That doesn't necessary mean that they are sick. However, feeders are usually kept in less than ideal and stressful conditions which promotes disease.
In your case, it could be anything; ammonia poison due to the sudden increase in bioload, improper acclimatization etc and without more details, all I can say is that your tank dimension is very unusual.
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