Setting up a ranchu tank, step by step
- Choose a wide, shallow container. About 75 liters (20 gallons) for the first fish and 40 liters (10 gallons) for each additional one. A 90 × 45 cm (36 × 18 in) tank filled to 30 cm suits two or three adults. A plastic tub of the same footprint works as well and costs far less. Check any container in the tank calculator.
- Keep the bottom bare or use a thin layer of fine sand. Gravel traps waste and ranchu can swallow it while grazing.
- Fit a gentle filter. A sponge filter on an air pump is the classic ranchu filter: cheap, safe for fry, and it adds oxygen. A hang-on or canister filter works if the outflow is baffled or spread with a spray bar.
- Dechlorinate and fill, then add an ammonia source (a pinch of food or bottled ammonia) and run the filter.
- Cycle the filter for four to six weeks until ammonia and nitrite both read zero within 24 hours of dosing. Moving a used sponge from an established tank cuts this to days.
- Add smooth decor only, if any. Ranchu tear their wen on sharp rocks and ornaments. Floating plants such as duckweed give shade and something to graze.
- Quarantine new fish in a separate tub for three to four weeks before they join the main tank.
Water parameters
| Parameter | Target | Why it matters |
|---|
| Ammonia | 0 ppm | Burns gills and skin. The top killer of new goldfish. |
| Nitrite | 0 ppm | Stops the blood carrying oxygen. |
| Nitrate | Under 20 ppm | High nitrate causes floating, stunting and poor colour. |
| Temperature | 65 to 75 °F (18 to 24 °C) | Stable matters more than warm. Avoid swings over 2 °F a day. |
| pH | 7.0 to 8.0 | Goldfish prefer slightly alkaline water. Keep it steady. |
| General hardness (GH) | 8 to 15 dGH | Very soft water slows growth and wen development. |
| Carbonate hardness (KH) | 4 to 8 dKH | Buffers pH so it does not crash between water changes. |
Use a liquid test kit rather than strips. It is the one piece of equipment that tells you what the fish cannot.
Feeding in brief
Feed sinking pellets or gel food as the staple, two or three small meals a day, each what the fish finish in about two minutes. Add frozen bloodworm, brine shrimp or daphnia a few times a week and blanched peas or greens once or twice a week. Never feed floating flakes: ranchu gulp air with them and end up floating. The full guide is on what ranchu eat.
A weekly routine
| When | Task |
|---|
| Daily | Feed, count the fish, watch them swim for a minute. Check they are level, eating and not flashing or clamping fins. Check the thermometer. |
| Weekly | Change 30 to 50% of the water with dechlorinated, temperature-matched water. Siphon the bottom. Test nitrate. |
| Every 2 to 4 weeks | Rinse the sponge or filter media in removed tank water, never tap water. Test pH and KH. |
| Monthly | Full test: ammonia, nitrite, nitrate, pH, GH, KH. Inspect the wen folds and fins closely. |
Seasonal care
- Spring: rising temperature triggers spawning. Males chase females hard. Separate the sexes or add spawning mops. See breeding.
- Summer: warm water holds less oxygen. Add an air stone, shade outdoor tubs, and keep the tank under 80 °F. Feed more; this is the growing season.
- Autumn: start reducing food as the water cools. Outdoor fish should go into winter well fed and healthy.
- Winter: below 60 °F, feed once a day or less. Below 50 °F, stop feeding outdoor fish. Indoors, a heater set as a floor at 65 °F prevents cold-water floating. See heaters.
Tank or tub?
Japanese breeders keep ranchu in shallow outdoor tubs and view them from above, which is how the breed was meant to be seen. Tubs give more surface per liter, cost little, and suit the fish. A glass tank lets you see the fish from the side and fits a living room. Either works if the water is shallow, wide and clean. For side-view Thai ranchu, a tank shows off the head and tail; for Japanese top-view fish, a tub shows off the outline.
Equipment checklist
- Tank or tub of about 75 liters per fish, wider than it is tall.
- Sponge filter and air pump, or a baffled hang-on filter.
- Liquid test kit for ammonia, nitrite, nitrate and pH.
- Water conditioner that removes chlorine and chloramine.
- Siphon and bucket kept only for the tank.
- Thermometer, and a guarded heater if the room drops below 60 °F.
- Sinking pellets or gel food, plus frozen food.
- Aquarium salt for treating minor problems.
- A second tub for quarantine and hospital use.
Mistakes that kill most ranchu
- Adding fish to an uncycled tank. Ammonia peaks in the first weeks.
- A tank that is too small or too tall. Ranchu need surface, not depth.
- Floating flakes and overfeeding. The main cause of floating and gut infections.
- Skipping water changes because the water looks clear. Nitrate is invisible.
- Fast tankmates such as comets and commons. See tankmates.
- No quarantine. Flukes and ich arrive on fish that look healthy.
- Medicating before testing. Most sick ranchu need cleaner water, not chemicals. See diseases.