Few plants sold for aquariums or fishponds are harmful to fish. However, many wild plants are poisonous or even fatal, so it is best to avoid picking your own plants to put into your pond or aquarium. As a general rule, purchase plants from a reputable dealer only and remember to buy water plants for ponds, underwater plants for aquariums-there is a difference.

Castor Bean, Castor-Oil Plant

Toxicity Rating: Extremely high. Death is likely with consumption of even a small amount of castor bean.

Dangerous Parts: The seeds are the primary source of toxin, but the rest of the plant may be slightly toxic as well.

Symptoms: Stomach irritation, abdominal pain and swelling, and digestive tract blockage.

Plant Description: This robust shrublike plant with reddish to purple stems may reach 12 feet in height. A perennial in the tropics, it is grown as a garden annual and the beans are processed commercially for oil. It originates in Africa and grows best in warm climates. In the northern United States, it is not unusual to see castor beans planted in public parks. The large (4 to 30 inches across), umbrellalike leaves have 5 to 9 pointed, fingerlike lobes. Long purple leaf stems grow near the centers of the leaf blades. Greenish-white or reddish-brown flowers appear in narrow, upright clusters. The fruit is a three-lobed, green or red capsule with a soft, spiny exterior. One large, mottled seed develops in each lobe. The shiny black-and-brown seeds look like large ticks.

Signs: The phytotoxin in castor beans is ricin, a water-soluble protein, which resides in the seed. Castor beans also contains ricinine, an alkaloid and an irritant. Commercially prepared castor oil contains none of these toxins. There is great species variability in susceptibility to ricin toxin, with humans and horses being the most at risk (25 grams of castor-bean seed is lethal for horses). However, it is best to keep this plant away from all ponds and fish. The seed is only toxic if the outer shell is broken or chewed open, which fish like to do. Seeds swallowed intact usually pass without incident. Signs of toxicity may not manifest for 18 to 24 hours after ingestion. The fish first shows signs of bloating. This toxin is closely related to the toxin in rosary pea.

First Aid: If the plant is near the pond, remove it immediately. Flush water to remove plant materials.

Prevention: Do not allow fish access to this plant. Do not allow seeds or any other part of the plant to be incorporated into fish feed. Don't grow castor beans near your pond.

Water Hemlock

Toxicity Rating: High. This is one of the most toxic plants in the United States.

Dangerous Parts: The roots contain the highest concentration of toxin, but all parts are toxic. Fish are liable to come in contact with tainted water or eat the roots.

Symptoms: Stomach irritation, abdominal pain and swelling, and digestive tract blockage. Death may be sudden.

Plant Description: One or more species of water hemlock can be found in wet fields and swampy ground all over the United States and Canada. The perennial stem of water hemlock may grow to 7 feet from its cluster of 2 to 8 fleshy or tuberous roots. Stems are smooth, branching, swollen at the base, purple-striped or mottled, and hollow except for partitions at the junction of the root and stem. A yellow, oily liquid smelling like parsnip exudes from cut stems and roots. Leaves alternately resemble a feather and are toothed, with the leaf veins extending to the leaf notches. Leaf petioles (leafstalk) partially sheath the stems. The small, white flowers are borne in flat-topped, umbrellalike clusters at the tips of stems and branches. Seed pods are small and dry with rounded, prominent ribs. Water hemlock grows in swampy areas and marshes, wet meadows and pastures and along stream banks and low roadsides.

Signs: Water hemlock is one of the most toxic plants in the United States, and a very small amount can cause death. Humans have died after only one or two bites of what they thought were parsnips (water hemlock root resembles a parsnip). Cattle are most affected (hemlock is also called cowbane), especially in early spring when the plants are smaller and more palatable and the roots are easily pulled up. Toxicity decreases during the growing season, and the toxicity of aboveground parts may be negligible when dry. The roots, however, are toxic at all times, even when dry.

The toxin is cicutoxin, a yellow, viscous resin that smells like carrots. Cicutoxin affects the central nervous system. The dose needed to cause clinical signs and the lethal dose are nearly the same, a little more than a gram of water hemlock per kilogram of body weight. For a fish, it doesn't take a very large dose. Once the fish has ingested even a small amount of the plant, signs will develop within an hour, sometimes as quickly as 10 to 15 minutes. Stimulation of the central nervous system begins, causing nervousness and dilated pupils. Fish will swell and plug up with death occurring within 30 minutes of the onset of signs.

First Aid: If water hemlock is found around the pond, remove it immediately. Flush the pond if necessary to remove any toxins. If fish appear bloated and lethargic, death is likely to ensue.

Prevention: Completely remove water hemlock, particularly the root, from areas near fishponds.

Poison Hemlock

Toxicity Ratings: Moderate to high.

Dangerous Parts of the Plant: All parts, especially young leaves and seeds.

Symptoms: The digestive tract will swell and plug up, and death will ensue.

Plant Description: This biennial herb came to the United States and Canada from Europe and grows as a weed. It can reach a height of 3 to 8 feet and has a smooth purple-spotted stem and triangular, finely divided leaves with bases that sheathe the stem. Fresh leaves and roots have a rank, disagreeable odor that is reminiscent of parsnips. Small but attractive white flowers arranged in umbrellalike clusters open in early summer. The fruit is tiny, flattened and ridged. Underground is a fleshy, unbranched white taproot. There are no hairs on the stems or leaves of poison hemlock and no branching, feathery bracts beneath the flower clusters. These plants are commonly found along roadsides, edges of cultivated fields, railroad tracks, irrigation ditches and stream banks and in waste areas.

Signs: The toxic components of poison hemlock include the volatile alkaloids coniine and gamma-conicine. Humans often fall victim, mistaking the roots for parsnips, the leaves for parsley, or the seeds for anise.

Affected fish show signs within 2 hours of eating the plant, tending to become bloated and plugged up. A mousy odor has been reported to emanate from affected fish.

Spring is the primary season for poison hemlock. At this time, the plant may also be more palatable. Toxicity increases throughout the growing season, and the roots become toxic only later in the year. Once dried, the toxicity is reduced but not eliminated.

First Aid: If poison hemlock is growing around the pond, remove it immediately. Flush the pond if necessary to remove any toxins. Observe fish for any indication of poisoning.

Prevention: Even tiny amounts of poison hemlock are lethal to fish. It is best to keep fish away from areas where poison hemlock grows. Poison hemlock may be difficult to eradicate.

Cherry (All Types)

Toxicity Rating: High.

Dangerous Parts: Damaged leaves pose the greatest risk. All parts are potentially toxic, and the pits are not safe.

Symptoms: Stomach irritation, abdominal pain and swelling, and digestive tract blockage. Death may be sudden.

Plant Description: Cherries may grow as a tree or shrub. The bark of young branches and twigs is scaly and reddish-brown with prominent cross-marks (lenticels). Leaves are alternate, simple, elliptic-pointed, leathery in texture, and finely toothed on the margins. Flowers are showy, fragrant and white, hang in drooping clusters, and produce dark-red to black cherry fruits. The wild black cherry commonly grows in fencerows, roadside thickets, and open woods.

Signs: Black cherry contains cyanogenic precursors that release cyanide whenever the leaves are damaged, such as by frost, trampling, drought, wilting or being blown down from the tree during storms. Fish that have access to the leaves may ingest toxic levels.

Healthy cherry leaves contain prunasin, a cyanide precursor that is normally nontoxic. When the leaves are damaged, the prunasin molecule splits, and free cyanide (also called prussic acid or hydrocyanic acid) is liberated. Many plants, especially those in the rose family, may potentially produce toxic levels of cyanide under certain conditions. Chokecherry (Prunus virginiana) is also toxic. There are reports of poisoning by peach sprouts, leaves and pits. Apricot pits and apple seeds are toxic as well.

Cyanide prevents the body from using oxygen at the cellular level, so although the fish continues to take in oxygen, their tissues and cells suffocate. After consumption, signs usually manifest within a few minutes but sometimes up to an hour may pass. Later, fish experience bloating and plugging of the digestive tract, which proceeds to a death within a few minutes to an hour.

First Aid: The clinical signs of cyanide poisoning tend to come on quite rapidly, and fish may be found dead without much warning. If the fish are exhibiting toxic signs, remove them from the pond and flush the water.

Prevention: Do not allow fish in a pond to have access to damaged cherry leaves, especially if they are hungry and there is no other feed available. The leaves and bark pose the greatest risk, but pits are not safe either. Never place fruit branches in aquariums.

Mushrooms

Toxicity Rating: Extremely high. One mushroom can be fatal.

Dangerous Parts: The whole mushroom is poisonous.

Symptoms: Stomach irritation, abdominal pain and swelling, and digestive tract blockage. Death may be sudden.

Plant Description: Few individuals have the special training needed to identify mushrooms accurately. For this reason, it is usually easier to assume an unknown mushroom is poisonous and remove it from the area.

Signs: Phallotoxins and amatoxins are the toxins responsible for fatalities in mushroom poisonings. When first ingested, the taste of the fungus may seem quite good to your fish. Even when death does not occur, the illness lasts several weeks and may do permanent damage to the fish.

Muscarine is a toxin that excites the parasympathetic nervous system, which results in stomach irritation, abdominal pain and swelling, and digestive tract blockage. Death may be sudden.

Muscimol, ibotenic acid, pantherin, tricholomic acid and related compounds are hallucinogenic substances that act on the central nervous system. They, and not the muscarine, constitute the psychoactive toxins in Amanita muscaria. When a large amount of A. muscaria containing these toxins is ingested, it often results in severe illness but fish usually recover.

Psilocybin and psilocina are well known compounds in the LSD family of hallucinogenic compounds (lysergic acid). They have a strong hallucinogenic effect on the central nervous system. Perhaps the most important factor is the presence of other toxic compounds in addition to the hallucinogenic agent in a given species. Species are common throughout the world and occur wherever cattle and horses are raised.

Monomethylhydrazine affects the central nervous system. The digestive tract swells and plugs up and death ensues.

First Aid: After eating a mushroom, a fish must be treated as soon as possible if it is to have a chance to recover. Remove it from the pond, and flush the pond.

Prevention: Because mushrooms can literally appear overnight, inspect your area on a regular basis and remove any mushrooms that you find.

Yew

Toxicity Rating: Extremely toxic, death is likely.

Plant Description: Several species of yew are planted as ornamental shrubs or hedges. They are woody perennials with flat, 1/2- to 1-inch-long evergreen leaves that are lighter green on the underside and broader than pine needles. The berry, technically called an aril, is grape-sized, juicy, and bright scarlet with a hole in the end, which makes it look cuplike. Dark evergreen needles make yew a beautiful plant in all seasons of the year.

Symptoms: Stomach irritation, abdominal pain and swelling, and digestive tract blockage. Death may be sudden.

Signs: The toxin is taxine, a mixture of alkaloids that slows down system conduction. As little as 0.1 to 0.5% of the fresh plant per body weight is lethal.

First Aid: First aid is usually impractical because fish die so quickly. If fish are still alive, remove them from the pond, and flush the pond.

Prevention: Never allow yew plants or trimmings within reach of ponds. It is a good idea to pick off the fruits of yew plants growing around homes where any type of animals, fish or children live.

Oleander

Toxicity Rating: High. Ingestion of even small amounts can kill.

Dangerous Parts: The entire plant is toxic.

Symptoms: Stomach irritation, abdominal pain and swelling, and digestive tract blockage. Death may be sudden.

Plant Description: Oleander grows as an indoor plant in the northern United States and as an outdoor shrub in California, Florida, and other warm regions. The thick, leathery leaves are lance-shaped and grow opposite each other. Leaves may sometimes grow in whorls. The leaves are 8 to 10 inches long, although smaller specimens have shorter leaves. Flowers are showy, approximately 1 to 3 inches in diameter, and grow in large clusters at the ends of the branches. They can be white or any shade of pink or red.

Signs: Oleander contains the toxins oleandrin and nerioside, which are very similar to the toxin in foxglove (digitalis). Oleander is not palatable, but will still be eaten by hungry fish. Dried or wilted leaves may be slightly more palatable than fresh leaves, but the leaves are still toxic when wilted or dried. Clinical signs may develop rapidly, and fish may be found dead with no prior warning. The digestive tract will swell and plug up, resulting in death. First Aid: The toxin acts quickly and is lethal in small amounts. Emergency measures should be used to remove the fish and empty the pond. Despite emergency care, the fish may still die, but the sooner treatment is begun, the better their chance for survival.

Prevention: Be able to identify oleander, and exercise extreme caution when near these plants. Oleander should never be planted where fish can have contact with it. Take extra care in cases where leaves can fall near or into a pond. Animals and humans can be hurt by oleander even without touching the plant. Breathing the smoke or burning branches can cause poisoning, and merely smelling the flowers may be harmful.

Rosary Pea

Toxicity Rating: High. Even one bean can kill.

Dangerous Parts: The beans are the primary risk.

Symptoms: Stomach irritation, abdominal pain and swelling, and digestive tract blockage. Death may be sudden.

Plant Description: This twisting perennial vine grows naturally in tropical climates. Rosary pea is established in certain areas of southern Florida. Leaves are alternate and compound, with 8 to 15 leaflets. Flowers are small and can be any shade from white to red to purple. The seed pod is about 1½ inches long and contains several seeds that are bright red with a black spot.

Signs: Toxic signs resulting from rosary pea ingestion are very similar to those of castor bean ingestion, except that rosary pea contains a more powerful toxin. One seed, if well chewed, can kill an adult human. The toxins in rosary pea are a protein called abrin and a glycoside called abric acid, which causes severe gastrointestinal signs. This progresses to weakness, shock and death within a short period of time.

Sometimes seeds are used to make jewelry and rosaries. If the seed is swallowed without damage to the seed coat, poisoning is unlikely, and the seed will tend to pass without incident. In cases where the seed coat is chewed or opened (as in drilling to make jewelry), toxic signs and death are likely.

First Aid: If your fish ingest rosary pea, there is not much you can do for them. Prevent further exposure and get the plant away from the area. Affected fish are likely to die.

Prevention: Rosary pea should never be allowed around the home for the safety of fish and humans alike. If jewelry or rosaries are made of rosary pea, discard them immediately.