Toxic Plants around your Aquaponics System

Keeping an aquaponics system in the backyard is a lot of fun but do keep a careful eye on the trees and shrubs your have growing around your system as they could be toxic.

Most plants are safe and toxic free but some species of flowering plants can be dangerous once the breeze catches them and they flutter down and land on your fish tank.

Here’s a first hand example.

Flowering Lillypilly

We have a concrete block fence with a border of Australian Lilypilly’s surrounding the outside fence for added privacy. Lilypillys are a dense green shrub and not considered poisonous but once a year, around this time in March they flower a profusion of tiny fine white blossoms. The wind will lay a fine carpet of white fine flowers across the patio, pool and aquaponics system over the course of a few days. In most circumstances the flowers die and dry up within a week.

We have our fish tank covered with timber slats but some of the fine flowers do make their way down to  float on the growbeds and even land on the fish tank’s water surface and sink.

The next day when feeding the fish I noticed that they were sluggish and drugged in appearance. They normally scoot away from the bright light when the hatch is opened but not today. They were “chillin’ out” and in no hurry  to go anywhere. I could have placed my hand under each fish and lifted them out of the water without a struggle. They appeared lethargic and not their normal sprightly selves.

What was going on?

Their water quality was within normal parameters. I looked at the carpet of blossoms and wondered if the plant might be toxic to fish? The following day the blossoms had stopped falling and the fish looked a lot happier. But amongst the group was one floater.  A nice plate sized Jade Perch had died. There were no discernible lacerations or stress markings on his body. But its a worrying lesson to watch carefully the plants you have growing around your tank.

Plant Toxins?

So what is going on? What toxins are dangerous to your fish?
The two primary chemicals that occur in most plants used for stunning fish are saponin and rotenone.


Saponins are glycosides with a distinctive foaming characteristic. When consumed by herbivores and mammals they breakdown in the digestive system and need to enter the blood-steam first in order to become toxic. But with fish – the story is different. The fish take in saponins directly into their bloodstream through their gills. The toxin acts directly on the respiratory organs of the fish breaking down the red blood cells.  Not always fatal, but its enough to stun the fish before fresh clean water arrives upstream to revive the fish. Native people around the world have used barks and plant leaves mixed into a paste and balls of the stuff thrown into the rivers to catch stunned fish for eons.

Plants that are poisonous to fish are a multitude. Some anecdotal evidence suggests that certain native Australian Eucalypts are hazardous to fish. Especially any gumnuts that fall into your growbeds and sump tanks. There’s a good reason to keep your system under cover for that very reason.

The ghost gum’s leaves were used by Aborigines to catch fish. Soaking the leaves in water releases a mild tranquilizer which stuns fish, making them easy to catch. An essential oil extracted from Eucalyptus leaves contains compounds that are powerful natural disinfectants and can be toxic in large quantities.


Rotenones are a different type of plant fish poison. Rotenones are an alkaloid toxin called flavonoids and stuns fish by impairing their oxygen consumption. Mainly found in Legumes  it is also used as an insecticide.

Native Folklore

South American Native Indians Poisoning Fish Source: Smithsonian

Botanist Professor Sir Ghillean Prance relates this story from an expedition to the Amazon with a native tribe of Indians in the 1960’s:

“The Maku Indians of the upper Rio Negro region of Brazil are well known for their fish feasts, where they go to a small river and catch a large number of fish by using fish poisons. The time I arranged to watch one of these, we were told that we must set out into the forest early in the morning. After two hours of a very fast walk we came to a small stream and I was glad to have arrived, but our leader said ‘not here’. We came to another stream an hour later just to be informed the same again. This process continued for about eight hours when finally the chief proclaimed that this was the correct stream.

Euphorbia Cotinifolia

We were almost too exhausted to observe the preparations as the men built a frame over the stream and placed their sacks of the fish poison leaves (Euphorbia cotinifolia). Meanwhile the women stirred up the muddy stream and the men began to beat the leaves so that the plant juices dripped down into the water. Very soon fish began to float to the surface and were gathered up by excited women and children.

We had a banquet as all the fish were roasted on fires and eaten. I asked the chief why we had to walk so far to carry out this operation. The answer I received was that they had poisoned fish in the first stream two moons ago, in the second five moons ago etc., until I got a complete description of when each stream had been used. He then informed me that if they poisoned a stream too frequently there would not be any fish left

A list of Poisonous plants that can kill fish.