Do ranchu get lonely, and are they aggressive?
A ranchu kept alone is not miserable, but a group of three is calmer and more active. And no, they are not aggressive. The chasing you see in spring is something else.
Alone or in a group
Goldfish are shoaling fish and ranchu are no exception. A single ranchu will eat, grow and live a long life, but it tends to be shy and spends more time resting. Two or more forage together, follow each other around the tub and come to the front faster at feeding time. If you have the space, three is a good number: it spreads any chasing and lets you feed evenly. Each extra fish needs its own 40 liters (10 gallons); see the tank calculator.
Are ranchu aggressive?
No. Ranchu are among the most placid goldfish. They do not bite, nip fins or defend territory. What keepers mistake for aggression is one of these:
- Spawning chases. In spring, males push females around the tank for hours, often into corners. It looks violent and can leave a female exhausted or with torn fins. Separate the sexes for a few weeks or add spawning mops so the eggs have somewhere to go. See breeding.
- Food competition. Fast fish bulldozing slow ones at the surface. The answer is sinking food scattered across the tub, and no fast tankmates. See tankmates.
- Nudging a sick fish. Goldfish investigate a tankmate that is lying on the bottom. It is curiosity, but move the sick fish to a quarantine tub anyway.
What normal looks like
A healthy ranchu cruises slowly at mid-water or along the bottom, tilts down to graze, and comes up to the front when you approach. Bursts of speed are rare. Resting on the bottom for a while after eating, or at night, is normal. Sitting on the bottom all day with clamped fins is not; test the water.