Japanese freshwater fish

White spot disease (Ich)

SignsWhite dots on body and fins, rubbing against stones, sitting at the bottom with fins clamped

PreventionAvoid sudden water-temperature drops in fall. Quarantine wild-caught fish in a separate container for 2 weeks before adding them

Cotton wool disease (water mold)

SignsWhite cotton-like growth on abrasions or the snout that spreads, loss of appetite

PreventionDo not scrape fish with a net. Avoid injuries during cold-water periods. Remove dead fish and leftover food immediately

Columnaris

SignsFin tips turn cloudy and rot away, snout turns white and ulcerated, gills turn white and rot

PreventionCaused by overcrowding and dirty water. Keep up water changes and prevent fights that damage fins

Hole-in-the-side disease (ulcer disease)

SignsRed, gouged-out holes on the flanks of crucian carp and bitterlings, scales fall off, common in spring and fall

PreventionOccurs readily at unstable temperatures of 15–20°C. Keep water temperature stable during seasonal changes

Anchor worm and fish lice

SignsThread-like or disc-shaped parasites on the skin, red swelling at the attachment site, thrashing and rubbing

PreventionCommon on wild-caught fish. Inspect the skin before introduction and isolate in a separate container for 1–2 weeks

Oxygen depletion / nitrite poisoning

SignsGasping at the surface, rapid gill movement, lying on one side, several fish dead in the morning

PreventionStream fish need a lot of oxygen. Aeration is a must in summer; check water quality weekly with test strips

Jumping out of the tank

PreventionDark chub and pale chub are strong jumpers. Always use a gap-free lid with a weight

First aidPick up with wet hands and return to same-temperature tank water. Support it near the aeration and do not touch it for several hours until it moves

Abrasions from nets or containers

PreventionWhen collecting, transfer fish in a bucket of water. Do not roll fish on the net. Do not touch with dry hands

First aidIsolate with a 0.5% salt bath and a slime-coat protectant. If white cotton starts to appear, treat for water mold. Feed only small amounts

Heat shock during collection or transport

PreventionCollect in the morning in summer. Keep water below 25°C with a cooler box and ice packs

First aidIf lying on its side, mix in cool water little by little, lowering the temperature over 1 hour. Aerate and keep dark and quiet

Torn fins from fighting

PreventionGive gobies one stone hideout per fish. Reduce crowding if male pale chub chase others

First aidIsolate the fish being chased and let the fins regrow in clean water. If cloudiness or rot appears, treat for columnaris

Nontuberculous mycobacterial infection (fish tank granuloma)

RouteThrough cuts on hands that touch tank water or fish

PreventionWear rubber gloves if you have cuts. Wash with soap after work; see a dermatologist if swelling develops

Salmonellosis

RouteHand to mouth after touching tank water, substrate or filter media

PreventionDo not pour tank water into the kitchen sink. Always wash hands after work

Gnathostomiasis / Metagonimus infection

RouteEating freshwater fish raw (larvae parasitize the fish)

PreventionNever eat wild-caught river fish raw. Cook thoroughly

Aeromonas infection

RouteDirty tank water entering a wound, eyes or mouth

PreventionDo not start a siphon by mouth. See a doctor if a wound becomes swollen and hot

Food

• Sinking pellets for river fish or goldfish as the staple, 1–2 times a day
• Give gobies more animal matter such as frozen bloodworms
• Pale chub and bitterlings also eat algae and plant matter. Feed only what is eaten in 2 minutes

Water

• Dechlorinate tap water and always match the temperature before adding
• Change 1/3 of the water weekly. Give stream fish a strong current and plenty of oxygen

Toys & play

• Create varied flow and hideouts with driftwood and stones
• Aquatic plants (hornwort, cabomba) give a calming space
• Gobies need places to burrow under flat stones

Nest, bed & hideout

• One hideout per fish. For territorial species, arrange them to block line of sight
• Bitterlings need freshwater mussels to breed, but mussels are hard to keep alive

Bedding & litter

• A thin layer of rounded river sand or fine gravel
• Siphon waste and leftover food from the bottom with a pipette or hose at water changes
• A bare tank is handy for isolation and treatment

Cage / housing material

Glass aquarium. Gap-free lid, filter, aeration and thermometer

Size & space

A 45 cm tank for 5–8 small fish as a guide. Crucian carp need 60 cm or more

Layout

• Point the filter outlet to ripple the surface and draw in oxygen
• For stream fish, current on the upstream side and hideouts downstream
• Create a calm, heavily planted area for bitterlings and crucian carp

Placement notes

• A sunny windowsill is dangerous in summer, with water temperatures above 30°C
• Place on a level, sturdy stand with an anti-seismic mat

Exercise, free-roam & walks

• No walks needed. Provide enough width to swim and a current
• Every morning, check the headcount, swimming and skin of every fish
• Do not chase with a net; move fish quietly in a bucket or cup

Temperature & humidity

Temperature10–25°C. Below 28°C in summer; no heater needed in winter, but avoid sudden changes

HumidityNot applicable underwater. Use a lid to prevent jumping and evaporation

Summer & winter

• Summer: prevent oxygen depletion with a cooling fan, air conditioning and stronger aeration
• Winter: indoor tanks can go unheated. Cut feeding to 2–3 times a week to prevent indigestion

Nail trimming

No nail care. Instead, weekly water testing (nitrite, pH) and water changes are the most important care

Brushing, skin & coat care

No coat care. To protect the slime coat, do not touch with bare hands and use a soft, fine-mesh net

Other care

Rinse filter media in tank water. Quarantine new fish in a separate container for 2 weeks, watching for parasites and white spot, before adding

Protection from predators

• Use a heavy lid so cats cannot reach into the tank
• Cover outdoor containers with netting against herons, crows, cats and dragonfly larvae
• Do not let outdoor containers breed mosquitoes. Keep the surface moving

Foreign-body ingestion

Fish may swallow small gravel that fits in the mouth and choke. Choose gravel larger than the mouth or fine sand, and match pellet size to the mouth

Being stepped on

No risk of being stepped on in the tank, but to avoid stepping on a fish that has jumped out, always check the floor after working with the lid open

Getting pinched

Small fish get caught in the filter intake or gaps around the heater. Fit a sponge on the intake and leave no narrow gaps in the layout

Falls

A tank tipping or falling is the biggest accident. For earthquakes, use an anti-seismic mat and lower the water level. Use a sturdy stand made for aquariums

Escape

Jumping out is the escape risk. Keep a gap-free lid on at all times. If found on the floor, return it to water even if it looks dry. Never release fish into rivers or ponds

Safe release procedure

1. Separate fighters with a divider that blocks line of sight
2. Move the fish being chased to a separate container of same-temperature tank water
3. For gobies, add stone hideouts and reduce density
4. After a fight, keep the water clean and let fins regrow

After the release

Torn fins regrow in clean water. If cloudiness, cotton-like growth or red swelling appears, treat as fish disease

What to tell the vet & what to bring

Collection site and date, tank size, number of fish, water temperature, water-change frequency, water test results. Bring the fish in a bag of tank water

Checking consciousness, breathing & heartbeat

• Are the gills moving? (normal is about 60–120 beats per minute)
• Does it flee when a finger approaches; do the eyes move?
• Even on its side, if the gills move there is a chance of recovery

Stopping bleeding

• Fin bleeding stops on its own in clean water
• Prevent infection with a slime-coat protectant and a 0.5% salt bath
• If bleeding from the body continues, see a vet who treats fish

Transport

• Seal the fish in a bag of tank water with plenty of air
• Carry in a foam box to keep the temperature stable, dark and without shaking