Your tank is overstocked and you should not add more fish. Fancy goldfish should have 10 gallons per fish. Single-tailed goldfish should have 20. Koi, which grow to 3", need hundreds of gallons. This is not just because of the size, but because of the bioload they have on the cycle. Do you know the nitrogen cycle? Here's an article http://www.aquariumadvice.com/articl...q=2&fldAuto=21

Mollys are actually a brackish fish, they do best in warmer partially salted water. Platies can be kept in colder water, but they like to eat the slimecoat off koi and goldfish. Minnows can be kept in coldwater. No sucker fish should ever be kept with goldfish.