Goldfish Tank Mates: Temperature, Size and Coldwater Companions
Why goldfish clash with tropical fish on temperature, how adult size and mouth fit endanger small fish, why fancy and single-tailed forms differ, and which coldwater companions sources describe.
Goldfish are coldwater fish whose optimum temperature, 20–22 °C according to Wikipedia, is below what most tropical species need. Adult goldfish eat tankmates small enough to fit in their mouths, and fast single-tailed forms outcompete slow fancy forms for food. The sources describe only a few companions, all of them coldwater fish: the weather loach in large tanks, mainly with single-tailed goldfish, and the rosy red minnow. The white cloud mountain minnow is advised against, even though it is often sold for this purpose.
Why goldfish and tropical fish conflict on temperature
Wikipedia gives 20–22 °C as the optimum for goldfish. It says temperatures below 10 °C are risky for fancy varieties, while commons and comets tolerate slightly colder water, and that very warm water above 30 °C can cause harm. Rapid temperature swings are dangerous at any level. Wikipedia describes goldfish as coldwater fish that are not suited to tropical aquariums and do well in unheated tanks. Tropical Fish Hobbyist adds a nuance: goldfish can cope with warmer water, and many tropical fish can live in the mid-70s °F (about 23–24 °C). The same article still recommends room temperature for goldfish and warns against extreme highs, because warm water holds less oxygen. Any shared range is therefore narrow, and heating a goldfish tank to suit tropical companions moves the goldfish away from its optimum.
Adult size and mouth fit
According to Wikipedia, wild-type goldfish usually reach 12–22 cm and can grow to about 40 cm, and single-tailed varieties often exceed 30 cm. Tropical Fish Hobbyist reports that common goldfish can grow beyond 45 cm and that even the smallest breeds reach 10–18 cm. Goldfish are omnivores, and the same article notes that larger goldfish have been known to eat smaller tankmates. Practical Fishkeeping says white cloud mountain minnows will mix with very small goldfish, but adult goldfish can eat them. It also places goldfish among the fish that will eat shrimp sooner or later.
Fancy versus single-tailed goldfish
Practical Fishkeeping describes straight-tailed goldfish as faster swimmers that can be boisterous, while fancy goldfish swim poorly and tend to wobble. Wikipedia explains the practical problem: commons, comets and other fast varieties can eat all the food at a feeding before slower varieties reach it. This can leave fancy fish stunted or starving, so the article advises keeping goldfish with similar body types together. Wikipedia gives about 20 US gallons (around 75 litres) per common or comet goldfish and about 10 US gallons (around 38 litres) per smaller fantail. Practical Fishkeeping puts the minimum tank length at around 60 cm. Goldfish have no stomach and produce a lot of waste, so they need a large volume of filtered water.
Coldwater companions described in sources
- Weather loach (Misgurnus anguillicaudatus): Seriously Fish states it tolerates 5–25 °C and recommends 17.8–23.3 °C, pH 6.0–8.0 and 1–12 dGH. It reaches up to 280 mm and needs a tank base of at least 120 × 45 cm. It is peaceful but may eat eggs, fry and very small fish. Seriously Fish says it may suit comets and shubunkins, but when active it can bother slow fancy goldfish. Practical Fishkeeping calls it a good choice for tanks of 90 cm or more. Seriously Fish's figure for long-term care is larger.
- Rosy red minnow (Pimephales promelas): Practical Fishkeeping calls it a fairly safe choice that stays at about 10 cm at most. Wikipedia gives an adult length of 7–10 cm and a preferred temperature of 10–21 °C, and notes that this colour form is sold mainly as feeder fish.
- White cloud mountain minnow (Tanichthys albonubes): Seriously Fish gives 14–22 °C, pH 6.0–8.5, 5–20 dGH and an adult size of 40 mm. It states that the species does not make a suitable companion for goldfish, despite often being sold as one.
Where parameters overlap
The goldfish optimum of 20–22 °C falls inside the recommended range for the weather loach (17.8–23.3 °C) and the white cloud minnow (14–22 °C). It overlaps the rosy red minnow's preferred range only at 20–21 °C. Temperature is therefore not what rules out the white cloud. The problem is that an adult goldfish can swallow it. Temperament and body shape, especially the needs of slow fancy goldfish, matter as much as water parameters.
Pond companions
For ponds, Wikipedia lists rudd, tench, orfe and koi as compatible with goldfish, and notes that koi need specialised care.
General stocking principles are covered in the fish compatibility guide. No pairing listed here is guaranteed, because individual fish differ and size relationships change as goldfish grow.