Hornwort

Quick facts

Max height

100 cm

Growth rate

fast

Difficulty

beginner

Placement

background, floating

Propagation

fragmentation

Water parameters

Temperature

5–30°C

pH

6.0 to 8.5

Hardness

5 to 25 dGH

Brackish

tolerated

Cold water

tolerated (unheated setups)

Light and nutrients

Lighting

low

CO2

not needed

Substrate

any

Feeding

feeds from the water column (use liquid fertilizer)

Substrate

What this plant roots into (or attaches to). The substrate affects both plant nutrition and water chemistry; see each linked page for full effects.

This plant feeds primarily from the water column, so substrate choice matters more for its fish-tank compatibility than for plant nutrition.

With fish

Plant-eating fish

will be eaten by mollies, silver dollars, large goldfish, and other plant-grazers

Diggers (corydoras, loaches)

fine - root system or attachment style handles it

Root-disturbing fish

tolerates fish that disturb roots

Habitat

Distributed across every continent except Antarctica. The species (Ceratophyllum demersum) is one of the most widespread aquatic plants in the world, found in ponds, lakes, slow rivers, and ditches from subarctic to tropical regions. Hornwort is a rootless plant: it floats freely in the water column or tangles loosely in substrate and other plants. The stems are covered in whorls of dark green, forked, needle-like leaves that give the plant a coarse, bristly texture. Stems can reach 2 m in the wild. The plant is a major component of natural freshwater ecosystems, providing shelter for fish fry, invertebrates, and aquatic insects. In the aquarium trade, hornwort is sold as a fast-growing, nearly indestructible oxygenating plant. It's also one of the most effective nutrient sponges among commonly available aquarium plants.

Outdoor pond use

This species transitions to outdoor ponds well, not just indoor aquariums.

Outdoor pond zones (USDA)

3 to 13 (winter low around -40°C or warmer)

Below the minimum zone, the plant won't overwinter outdoors but can still be grown seasonally and overwintered indoors. Several pond-friendly species (water hyacinth, water lettuce, parrot's feather) are regulated as noxious in some jurisdictions; check the legality data on the profile before releasing anything to an outdoor body of water.

Care notes

Virtually indestructible. Grows in any light (low to high), any temperature (10–30°C), any water chemistry (pH 6.0-8.5, soft to very hard water), and without CO2. Growth is fast: under moderate light, stems elongate 5–15 cm per week. Under high light with nutrients, growth is aggressive and requires frequent thinning. The plant can be weighted and planted in substrate (where it doesn't root but stays anchored by gravity) or left floating. Floating hornwort grows fastest and provides excellent shade and fry cover. The main drawback is needle drop: hornwort sheds its fine leaves when conditions change (especially temperature fluctuations or low light), creating a mess of tiny green needles that clog filters and coat the substrate. This shedding is temporary during acclimation but can be chronic in marginal conditions. Hornwort is allelopathic: it releases chemicals that inhibit the growth of some algae and competing plants, which is an advantage for algae control but can affect nearby sensitive species. Propagation is by fragmentation; any stem piece with a few whorls of leaves grows into a new plant. In aquaponics, hornwort is a useful floating nutrient filter that's nearly impossible to kill. Trim excess growth aggressively and often.

Further reading

• CO2 injection for planted tanks: what it costs, what it does, and when to skip it
• How to cycle a fish tank