Terrestrial Plants with Roots in the Aquarium

How pothos, philodendron, lucky bamboo and peace lily are grown with roots in tank water, what studies show about nutrient uptake, and their toxicity to pets.

Overview

Some aquarists grow terrestrial or semi-aquatic houseplants with their roots in the aquarium water and their leaves in the air above the tank. Golden pothos (Epipremnum aureum), philodendrons (Philodendron), lucky bamboo (Dracaena sanderiana) and peace lilies (Spathiphyllum) come up most often. The idea is the same as in aquaponics. Fish waste is rich in nitrogen and would build up to harmful levels if nothing removed it. The plants take up these nitrogen compounds and use them to grow.

The plants most often used

• Golden pothos (Epipremnum aureum). According to Wikipedia, it grows quickly in hydroponic culture and is often set on top of an aquarium so it can root into the water. It was originally endemic to Mo'orea in the Society Islands. It now grows wild in many tropical countries and can become strongly invasive where it is not native. Florida listed it as an invasive species in 1999.
• Philodendrons (Philodendron). They grow mainly in humid tropical forests but also in swamps and on river banks. Their aerial roots both anchor the plant and gather water and nutrients. Wikipedia notes that Philodendron oxycardium can be grown indoors in containers of water.
• Lucky bamboo (Dracaena sanderiana). It is not a true bamboo and belongs to a different taxonomic order. It is native to Central and West Africa and is sold widely under the name lucky bamboo.
• Peace lilies (Spathiphyllum). In the wild they grow on the floor of humid tropical forests, in shady, moist or wet places along rivers and streams. They often form colonies along waterways, sometimes in spots that flood from time to time.

Setting up: roots wet, foliage dry

The usual layout keeps only the roots, and sometimes the lowest part of the stem, under water. The leaves stay above the surface. Plants can sit in pots set into holes in a lightweight lid, in a holder on the rim, or in a hang-on-back filter, kept away from the motor and impeller so the roots cannot block it. Growing shoots can be trained up a wall or a shelf.

1. Wash all potting soil and fertiliser off the roots before putting the plant in the tank, so neither gets into the water.
2. Hold the plant in place with small pebbles or an inert support instead of soil.
3. Keep the leaves out of the water and let only the roots hang into the tank.
4. Make sure the water is well circulated and aerated. Submerged roots need oxygen, and poorly oxygenated roots are prone to infection by pathogens. Standard aquarium filtration and aeration supply enough oxygen.
5. Give the plants enough light, for example a lamp on a timer that keeps a steady day–night cycle.

Evidence for nutrient export

No peer-reviewed study measuring nitrate removal by houseplants in a home aquarium was found among the sources consulted. The best available evidence comes from wastewater and eutrophic-water studies that used the same ornamental species or their close relatives:

• Constructed wetland microcosms planted with Anthurium sp., Zantedeschia aethiopica and Spathiphyllum wallisii and run for nine months removed 28–44% of nitrates and 25–45% of phosphates. Units without plants removed roughly 40–50% less pollution (Sandoval et al., 2019, Int J Environ Res Public Health).
• In a 12-month mesocosm wetland study, Spathiphyllum blandum improved removal of NH4-N by 16% and PO4-P by 29% compared with unplanted controls. It also lowered the rate of denitrification (Sandoval et al., 2020, Water Sci Technol).
• In a 39-day lab test, floating beds of Spathiphyllum floribundum, Chlorophytum comosum, Peperomia obtusifolia or Hydrocotyle sibthorpioides survived in artificially eutrophied water and gained biomass. Ammonium-nitrogen removal was about 97% (Zhang et al., 2021, Bull Environ Contam Toxicol).
• When watered with untreated brewery wastewater, golden pothos and spider plant were among the plants with the best growth and survival. The authors judged that they could be sustained without extra nutrients (Wolcott et al., 2023, Environ Technol).

These systems differ from an aquarium in plant density, retention time and nutrient load, so their percentages cannot be applied directly to a tank. They show that these species can take up nitrogen and phosphorus from water, not how much a few cuttings will remove from a particular aquarium.

Toxicity

The ASPCA lists golden pothos as toxic to dogs and cats. Its toxic principle is insoluble calcium oxalates, and signs include oral irritation, intense burning of the mouth, tongue and lips, heavy drooling, vomiting and difficulty swallowing. The ASPCA gives the same toxic principle and signs for peace lilies (Spathiphyllum). In people, Wikipedia reports that eating pothos can cause burning and swelling in and around the mouth, and that handling it can cause contact dermatitis. Philodendrons contain calcium oxalate raphides at up to about 0.7%. In one study of 127 children who ate philodendron, only one, aged 10 months, had a minor symptom (slight swelling of the upper lip). Older reports of severe poisoning in cats were not reproduced in a controlled 1978 experiment, so the evidence for cats is mixed. The ASPCA lists Dracaena species as toxic to dogs and cats because they contain saponins. Signs include vomiting (sometimes with blood), depression, loss of appetite, drooling and, in cats, dilated pupils.