How big do ranchu get?
Five to eight inches of body, most of it in the first two years. Thai jumbo lines go bigger. Tank size does not cap growth the way people think; water quality does.
Size chart
Ranchu are measured by body length, nose to tail joint, not including the tail. Roughly:
- 3 months: 1 to 1.5 in (2.5 to 4 cm).
- First year (tosai): 2 to 4 in (5 to 10 cm).
- Second year (nisai): 4 to 5.5 in (10 to 14 cm).
- Adult (oya, 3 years and up): 5 to 8 in (13 to 20 cm) of body, and about an inch a year after that, slowing to nothing.
- Thai jumbo lines: 8 to 10 in (20 to 25 cm) body, with the tail adding more. These are the "giant ranchu" sold by importers.
Weight follows the round body: a full adult can weigh 300 to 500 grams, far heavier than a common goldfish of the same length.
Growth rate
Almost all of the length comes in the first eighteen months. A fry in a warm, green outdoor tub with live food several times a day can reach 3 inches by autumn. After the second year, growth is slow and the fish adds width, depth and wen rather than length. This is why breeders push tosai hard and why a fish bought as an adult will not get much larger no matter how it is fed.
What limits size
- Water quality. Nitrate and hormones building up in a small, under-changed tank stunt goldfish. Big water changes remove both. A ranchu in a small tank with daily changes outgrows one in a big tank with monthly changes.
- Temperature. Growth is fastest in the low to mid 70s °F. Cold water stalls it.
- Protein. Bloodworm, brine shrimp and a 40%+ protein pellet build fish. See what ranchu eat.
- Genetics. Japanese show lines are bred small and compact; Thai lines are bred big. A fish will not exceed what its line allows.
Planning the tank
Size your tank for the adult, not the fish in the bag. An 8-inch ranchu needs 75 liters (20 gallons) and a good stretch of surface on its own. Run your numbers through the tank calculator.