I have a lone goldfish, a comet, about 10 cm long from tip to tail. I want to buy a second fish and am wondering about size. Is it ok to introduce a much smaller goldfish, or maybe three smaller.
Minimum Tank Sizes for GoldfishFirst Common Goldfish 40+ gallonsA lone goldfish often becomes lethargic, hides frequently, and may refuse food.

This particular example perfectly highlights why Suitable Tank Size For Lone Goldfish is so captivating.
Tank Size Requirements. The most common mistake made when keeping goldfish is keeping them in tanks too small for their needs. Goldfish are NOT suitable for bowls. They need much more space to thrive.

Oranda Goldfish Tank Size & Specifications. Your tank should be at least 20 gallons (or bigger if possible).Are Oranda Goldfish Lone or Societal In Nature? Oranda Goldfishes do get lonely when left alone.

Our Golden Rule for Tank Size. Forget the old one inch of fish per gallon rule; it simply doesnt work for messy fish like goldfish. Here is a much safer and more humane guideline: Start with a 20-gallon (75-liter) tank for your first fancy goldfish.
Factors Influencing Tank Size. Ideal Tank Sizes for Different Scenarios. Beyond the Dimensions: Tank Shape and Orientation.