Coldwater Native Fish Aquarium: Temperate Species Without a Heater

Keeping temperate native fishes in an unheated tank: darters, shiners, sticklebacks and sunfish, collection rules, the oxygen-versus-heat problem in summer and seasonal breeding cycles.

The unheated temperate aquarium

A coldwater native aquarium keeps fishes from the temperate zone of the keeper's own region in a tank without a heater, so that water temperature follows the room and, to some degree, the season. Tropical Fish Hobbyist groups North American candidates by habitat: darters that live in cool, fast-moving riffles high in dissolved oxygen; shiners that school in the open water of clear creeks; killifish and topminnows from quiet backwaters, pools and swamps; and small sunfishes. General care of unheated tanks is covered in coldwater-aquarium-guide; the flow hardware used for riffle species is described in hillstream-river-manifold-tank-guide.

Legality of collection

Rules on catching and keeping wild fish differ by jurisdiction, and the sources describe several models. In Virginia, the Department of Wildlife Resources requires a freshwater fishing licence for collecting fish by any listed method, allows 20 nongame fish and aquatic invertebrates for personal use, permits hand capture, hook and line, seines no larger than 4 by 10 feet, umbrella nets up to 5 by 5 feet, minnow traps with throat openings of at most 1 inch, and cast nets, and prohibits possession of threatened and endangered species and of the candy darter. In Missouri, according to the Department of Conservation, a valid fishing permit allows possession of 100 non-game fish, endangered and threatened species may not be collected or possessed, and game species may be taken only by hook and line or as regulations permit; a seine of 10 by 5 feet with 3/16-inch mesh is described as adequate for small fish in shallow pools.

Tropical Fish Hobbyist adds that many small natives count as bait fish and may be collected with a standard fishing licence in some states, while other states are very restrictive; the bluenose shiner (Pteronotropis welaka) is listed as a Species of Special Concern in Florida and may not be collected. The centrarchid article warns that some sunfishes are classed as game fish and may not be taken with nets. Both TFH pieces advise collecting only what is needed, preferring sub-adults, and never releasing aquarium fish into the wild.

Candidate species groups

Darters (Etheostoma): Wikipedia counts 146 species in 18 subgenera, native to North America and mostly restricted to the United States. The rainbow darter (Etheostoma caeruleum) reaches 2–3 inches (51–76 mm), lives in clean, rocky riffles with swift current, holds position on the substrate in flowing water, feeds on small invertebrates such as caddisfly larvae, and is very sensitive to pollution and silt. TFH also names the redline darter (E. rufilineatum), the blueside darter (E. jessiae, about 3 inches) and the greenbreast darter (E. jordani, seldom over 2.5 inches).
Shiners: the rainbow shiner (Notropis chrosomus, now placed by Wikipedia in Hydrophlox) measures 5–8 cm, inhabits clear, low-turbidity headwater streams over gravel and sand and spring runs, and is kept in aquaria and ponds. TFH lists the tricolor shiner (Cyprinella trichroistia) and the metallic shiner (Pteronotropis metallicus) as schooling fish for larger tanks.
Sticklebacks: the three-spined stickleback (Gasterosteus aculeatus) is native to inland and coastal waters north of 30°N, matures at 3–4 cm and occasionally reaches 8 cm, lives in fresh, brackish and salt water, and is described by Wikipedia as easy to find in nature and easy to keep in aquaria.
Sunfishes: TFH describes pygmy sunfishes (Elassoma) that seldom exceed 1–1.5 inches and need species tanks with cover such as oak leaves or Java moss, and Lepomis species from the orange-spotted sunfish (about 4 inches) to bluegill (about 14 inches). Most Lepomis males dig saucer-shaped nests and defend territories vigorously; the pumpkinseed, longear and dollar sunfish are singled out as aggressive, the warmouth and redear as calmer.

Oxygen and the summer heat problem

The weak point of an unheated tank is not winter but summer. The UF/IFAS fact sheet on dissolved oxygen states that water at 45 °F (about 7 °C) holds 11.9 mg/L of oxygen at saturation while water at 90 °F (about 32 °C) holds 7.4 mg/L, that 5 mg/L or more is recommended for fish health, that most species are distressed at 2–4 mg/L and die below 2 mg/L, and that warm water also raises the metabolic oxygen demand of fish. Riffle fishes such as darters come from water that Wikipedia describes as continuously aerated by turbulence, so a heatwave in a still room tank hits them twice: less oxygen dissolves, and they need more of it.

The same fact sheet lists the main causes of oxygen crashes: reduced photosynthesis in cloudy weather or after an algae die-off, decomposition of excess organic waste, and stratification that stops water mixing; night is the critical period, because plants and algae respire without producing oxygen, and mechanical aerators run on timers from about 10 pm to 8 am are the standard countermeasure in pond culture. For an aquarium this translates into strong surface agitation or an air pump, restraint in feeding, and, for riffle species, the unidirectional flow described in the hillstream guide. The Missouri Department of Conservation notes that more aquarium fish die from ammonia build-up than from any other cause and suggests feeding once every two days when a tank is new.

Seasonal cycles and breeding

Temperate fishes are tuned to an annual cycle, and the sources record it species by species. The rainbow shiner spawns in late spring and early summer at 16–25 °C, from April through June or July according to TFH, and Wikipedia notes that it deposits eggs in the gravel nests built by Semotilus and Nocomis chubs; breeding males develop blue ventral fins, a purple head and a red snout. The rainbow darter mates in spring when water reaches 17–18 °C, and a female lays about 800 eggs over several days. Male three-spined sticklebacks build a nest from plant material glued with spiggin, a kidney secretion, in 5–6 hours, turn bright red at the throat with blue eyes, and fan the eggs, which hatch in 7–8 days at 18–20 °C; many populations mature in two years and breed only once before dying.

Among sunfishes, TFH states that pumpkinseeds benefit from a wintering period in which the water is allowed to cool and food is offered sparingly. Letting the tank track the seasons, cooler and leaner in winter and warmer with longer days in spring, is therefore part of husbandry rather than a fault of the unheated set-up.

Feeding and husbandry notes

Darters are insectivores (Wikipedia); TFH describes pygmy killifish and pygmy sunfish as preferring small live foods, and sunfishes as taking frozen bloodworm, mysis and chopped krill.
Overfeeding is a documented risk: TFH limits bloodworm to about one third of the diet for the orange-spotted sunfish and advises sparing use for the bantam sunfish.
Sticklebacks feed on chironomid larvae and amphipods on the bottom or on plankton in open water (Wikipedia).
When adding wild-caught fish, the Missouri Department of Conservation advises matching the tank temperature to the water they came from.
Fish from riffles need current and cobble; fish from backwaters need cover and still water. TFH recommends species-specific tanks for the smallest groups.