Fish tank temperature: the right range for 16 common fish

Tropical or cool water?

Start by sorting the fish into two groups, because they need opposite equipment. Most of the fish sold in a pet shop's tropical section come from warm water, and RSPCA Australia is blunt about what that means: heating is essential, normal room temperature is generally too cold, and a submersible heater is the only way to hold a constant temperature. It recommends 24–26 °C (75–79 °F) for tropical fish.

A short list of popular fish want the opposite. The RSPCA gives goldfish 10–21 °C (50–70 °F) and says a heater is not normally necessary; OATA's goldfish care sheet allows 4–25 °C and adds that goldfish don't tolerate extreme fluctuations. Seriously Fish lists the white cloud mountain minnow at 14–22 °C (57–72 °F) and warns that permanent exposure to warmer conditions is likely to shorten its life. Cool-water fish and tropical fish don't belong in the same tank.

Temperature chart: 16 common fish

Ranges below are copied from each species' Seriously Fish profile (goldfish: the RSPCA), in °C as published, with °F rounded to the nearest degree. They are the limits the source considers suitable, not targets to sit at the edge of: a tank set at the very top of a range has no margin for a hot afternoon.

Three things stand out. Guppies have the widest range here, 17–28 °C. Discus (26–30 °C) and German blue rams (27–30 °C) sit at the warm end, and Seriously Fish says the ram isn't recommended for the general community aquarium. And two fish that look interchangeable in the shop can differ by several degrees: the neon tetra tops out at 25 °C, while the cardinal tetra goes to 29 °C. Each fish name links to its tank-size page.

Find the temperature your fish share

In a community tank the right setting is the band every species' range includes. Its floor is the highest minimum among your fish and its ceiling is the lowest maximum. Tick the fish below to find it. The RSPCA's instruction is simple: research the temperature needs of every species you want to keep.

Aim for the middle of the shared band rather than an edge, so a heater that drifts a degree or a warm week doesn't push one species out of range. If the checker finds no overlap, the fix is a different fish, not a compromise temperature. The aquarium stocking calculator checks tank size, schooling and temperament as well.

Why warmer is not safer

It is tempting to run a tropical tank warm, on the theory that tropical fish like heat. Water chemistry argues against it. Warm water holds less dissolved oxygen: a standard saturation table puts pure water at about 9.2 mg/L at 20 °C, 8.4 mg/L at 25 °C and 7.7 mg/L at 30 °C, roughly a sixth less at 30 than at 20. Meanwhile the fish, and the bacteria in the filter, use oxygen faster as the water warms. The RSPCA describes the result: in water that is too hot, oxygen falls and fish gasp at the surface. Warmth also shifts ammonia toward its more toxic un-ionised form, which our high ammonia page covers.

Cold has its own signs. The RSPCA notes that tropical fish in water that is too cold swim slowly. A tank that has gone cold after a heater failure should be brought back to its normal setting steadily, not shocked with hot water.

Sudden changes

A steady temperature matters as much as the number. The RSPCA warns that new water at a different temperature from the tank will shock the fish, and recommends bringing it to tank temperature first. It also says not to put a tank by a window, which can raise the water temperature dangerously high. RSPCA Australia notes that room temperature fluctuates, which is stressful for fish.

Swings matter for disease too. UF/IFAS observes that freshwater ich outbreaks often appear when water temperatures are changing more rapidly, and that temperature controls the parasite's pace: at 75–79 °F its life cycle completes in about 3 to 6 days, and at cooler temperatures it is prolonged. That is why treatment timing in our ich guide depends on the tank's temperature.

Heater and thermometer

• A thermostat-controlled heater. The RSPCA says all heaters must be controlled by a thermostat. Size it to the tank and the room with the heater wattage calculator; an oversized heater that sticks on heats a small tank fast.
• A thermometer you actually read. Equipment fails, so the RSPCA recommends a digital thermometer and checking the temperature every day.
• No heater for cool-water fish in a normal room. For goldfish in a cold room, the RSPCA suggests a heater set to the bottom of their range.
• No bowls for tropical fish. RSPCA Australia points out that small bowls are too small to fit a suitable heater.

Saltwater and reef tanks

Marine fish and corals work to a different, tighter brief. Our reef tank temperature page covers the 76–81 °F target, what the published ranges say, and how fast to change it; the quarantine tank guide covers heating a small hospital tank without cooking it.