Grass carp

Water parameters

Minimum tank: 2000 L per individual at harvest size.

Feed and growth

Feed protein

25% target

Daily feed (warm)

3.00% of body weight

Daily feed (cool)

0.80% of body weight

Max density

35 g per litre

A 15000 g adult eats about 450.0 g of feed per day at optimum. 10 fish at adult size: ~4500 g daily.

Legality

Rules vary by jurisdiction and change over time. Verify with your local fisheries or wildlife authority before stocking.

Unlisted jurisdictions default to "check local regulations".

Origin and habitat

Ctenopharyngodon idella is a large, herbivorous cyprinid native to the rivers of the Pacific Far East, from northern Vietnam north to the Amur on the Russia-China border. It feeds almost entirely on aquatic and bankside vegetation, taking anywhere from a fifth to its full body weight in plants each day. Adults grow big, to about 1.5 metres and more than 40 kg, though farmed fish are usually harvested at one to five kilograms. Grass carp is one of the four domesticated Chinese carps, with silver, bighead, and black carp, and it is the most produced freshwater fish in the world, well over five million tonnes a year, the bulk of it from China. It has been sent to many countries for aquatic weed control, and it has turned invasive in several, notably the Mississippi basin, where it now reproduces and has spread across some 45 US states.

Climate and outdoor ponds

Climate

temperate (handles seasonal swings)

USDA zones

3–12 (winter low around -40°C or warmer)

Heating needed

no

Cooling needed

no

Care notes

A herbivore that can cut feed costs in aquaponics by eating plant waste directly, duckweed, vegetable trimmings, grass clippings, and nuisance aquatic weeds, which makes it unusually efficient in systems with plenty of plant biomass. It prefers warm water, growing best around 25–30°C within a tolerated range of about 10–33°C, and it overwinters in cold climates. On vegetation alone it reaches one to three kilograms in twelve to eighteen warm months; a supplemental pellet of 25 to 30 percent protein speeds growth, with feed conversion around 1.5 to 2.5, but the real value is turning low-cost plant matter into fish. Stocking runs 10 to 20 g/L. The big constraint is regulation: because of its invasive potential, many places allow only triploid, sterile grass carp under permit, a sterile form first produced commercially in Arkansas in 1983, while California, Washington, Oregon, and others ban it outright and Australia prohibits it entirely. Check local rules before buying, and source certified triploid fingerlings where required. The flesh is white and acceptable but bony. In aquaponics it works best as a plant-waste recycler in polyculture with another species such as tilapia rather than as the main harvest fish.

Further reading

• Aquaponics water chemistry: balancing pH for fish and plants
• Fish for aquaponics: tilapia vs everything else