Why Use Botanicals in Your Aquarium?
The science and soul behind natural aquarium keeping
Ask a conventional aquarium keeper what they want their tank to look like, and most will say: clean. Clear water. No debris. A pristine glass box where everything is controlled and tidy.
Ask someone who keeps a botanical tank, and you'll get a different answer entirely.
They want dark water. Decomposing leaves on the substrate. Seed pods softening into biofilm. A tank that looks, in some meaningful way, like the bottom of a flooded Bornean forest or an Amazonian igapó.
So what actually happens when you add botanicals to an aquarium? What are tannins, and why do they matter? And why do hobbyists who try the botanical method so rarely go back to bare tanks?
Six Reasons the Science and the Hobby Agree
01 — Water Chemistry Closer to Where Fish Evolved
Here's something beginner guides rarely mention: your tap water is very different from the water many tropical fish come from.
Tap water is treated to be safe for people. It is usually low in dissolved organic matter and often moderately hard. The blackwater rivers of the Amazon, the peat swamps of Borneo, and the slow, leaf-littered streams of Southeast Asia are the opposite: soft, acidic, and stained by humic substances released from decomposing plant matter [1].
When leaves, seed pods, bark, and wood break down in your aquarium, they release tannins along with humic and fulvic acids. These are the same kinds of compounds that shape the habitats of wild bettas, many tetras, Apistogramma, and countless other aquarium fish. In soft water they gradually lower pH and tint the water. In hard, well-buffered water the pH may barely move, but the tannins and humic compounds are still there.
These compounds do real work for fish from these habitats. In the soft, acidic water of the Rio Negro, dissolved humic substances help fish take up sodium and calcium and hold their salt balance at pH levels that would otherwise stress them [1].
This isn't just about looks. It's water chemistry that works with your fish rather than against them.
02 — Tannins and Microbes: Promising Science, Long Tradition
Tannins are polyphenols, the same family of compounds plants use to defend themselves against microbes and pests. When botanicals break down, some of these compounds end up in the water.
The research on Indian almond (catappa) leaves is encouraging:
- In lab tests, catappa leaf extract stopped the growth of Aeromonas hydrophila, a common fish bacterium [5].
- Bettas immersed in catappa extract for three days had higher survival, and their white blood cell profile shifted when they were later exposed to Aeromonas [3].
- In tilapia, catappa leaf extract boosted several measures of innate immunity and reduced deaths after a bacterial challenge [4].
- Reviews of dissolved humic substances report that they can help fish recover from stress-related damage and act against some fungi, including the water mold Saprolegnia [1][2].
Keep in mind that most of these studies used concentrated extracts or controlled doses. A few leaves in a tank give much lower and more variable levels, so think of botanicals as support for a healthy tank, not a treatment.
The tradition behind them is long. Breeders across Southeast Asia have used catappa leaves, peat, and bark for generations, especially in betta breeding tanks. Even the scientists who first bred the Critically Endangered Betta hendra in captivity conditioned every tank with dried catappa leaves [6].
03 — Less Stress, More Natural Behavior
Stress is one of the quiet problems of home aquariums. A stressed fish eats less, hides more, shows duller colors, and is more vulnerable to disease. The effects build over time.
Botanicals help in two ways.
The chemistry. The science here is still developing. Reviews describe humic substances as a mild stressor that prompts fish to switch on their own defenses, a bit like exercise strengthens a body [1]. In aquaculture, they've been linked to better condition and faster recovery from stress-related damage, although researchers say the mechanism isn't fully understood [2].
The environment. This is where botanicals clearly shine. Tinted water, leaf litter, root tangles, and seed pods create the shaded, sheltered spaces most small tropical fish seek out. In the wild, a small fish in open, bright water is an easy target. Give it cover and it behaves very differently.
Keepers see the shift often: shy species come out of hiding, schooling fish settle, and colors deepen. It's the difference between a fish surviving in captivity and living something closer to the life it was built for.
04 — Biofilm: The Hidden Food Web
As botanicals break down, they're colonized by biofilm: thin, living films of bacteria, fungi, and microorganisms that coat every submerged surface. To the uninitiated, it looks like a problem. That faint white fuzz on a new catappa leaf? The thin film on a piece of wood?
That's food.
In many tropical freshwater streams, biofilm and the detritus it grows on sit near the base of the food web. Fry pick at it in their first days, shrimp graze it constantly, and small fish spend hours foraging through leaf litter for the tiny animals that live in it.
A botanical tank with established biofilm gives your animals something to forage between meals. Fry get supplemental grazing, shrimp have food on every surface, and the tank develops a small, living ecology that no bottle of fish food can copy. Regular feeding is still essential; biofilm adds to it, it doesn't replace it.
05 — Spawning Cues and Breeding Behavior
Many tropical fish breed with the rainy season, when rising water floods the forest floor, washes in leaf litter, and darkens the water with tannins. Softer, darker water and a surge of organic matter can signal that conditions are right.
A botanical tank can send a similar signal.
Hobbyists regularly report fish showing breeding behavior after moving into a mature botanical tank: deeper colors, courtship, territorial displays, and eventually spawning. Some of these are species that never spawned for them in bare tanks, such as wild bettas, Apistogramma, tetras, and corydoras. Results vary with species, water, and diet, but botanicals often help tip the balance.
Botanicals also provide the structure many fish need to breed. Apistogramma females look for tight, enclosed caves to lay their eggs. Wild bettas build bubble nests under broad leaves; in the Betta hendra breeding study, males nested beneath dried catappa leaves [6]. Many egg-scattering fish lay their eggs among leaf litter and fine plant material.
06 — A Living, Evolving Ecosystem
There's a deeper benefit experienced hobbyists mention again and again: the tank comes alive in a way other setups don't.
A botanical tank isn't a static display. Leaves soften and break down. Pods slowly settle into the substrate. Biofilm communities shift as you add or remove botanicals, new microfauna appear, and the water chemistry finds its own balance over time. Your fish respond, foraging through fresh litter, investigating new pods, and spending more time out in the open as their confidence grows.
This isn't a tank you set up and walk away from. It's one you watch, learn from, and adjust with.
A Word of Honesty
Botanicals aren't magic. A handful of catappa leaves won't fix poor water quality or make up for overfeeding, and tannins don't replace good filtration, water changes, or ammonia and nitrate control. Everything here assumes a responsibly run aquarium with the basics in place.
Add botanicals gradually, too. In very soft or RO water, a large load of fresh leaves can drop pH faster than your fish can adjust, so test as you go.
Used thoughtfully, botanicals shift a tank from a container of water with fish in it toward something closer to a working ecosystem. For the fish in your care, that can be the difference between surviving in captivity and living something closer to the life they were built for.
Where to Start
You don't need a complex setup to begin. A few Catappa Leaves on the substrate, some Casuarina Cones, and a little Organic Sphagnum Peat Moss in your filter will start conditioning your water today. Remove activated carbon from the filter, since it pulls tannins out of the water. Then watch, and let the tannins build.
The water will darken.
The leaves will grow a faint biofilm.
Your fish will begin to relax.
That's not a problem. That's the beginning of something better.
Ready to begin? Browse our leaves, seed pods and cones, and roots and bark, then head to The Sunken Forest for species spotlights, biotope guides, and setup inspiration.
This article is for educational purposes. Always monitor water parameters and livestock behavior when introducing botanicals. Botanicals are not for human consumption and require preparation before aquarium use. Because they're natural products, size, shape, color, and texture vary from piece to piece.
Where Sunken Forests Come to Life.
Sources
- Steinberg, C. E. W., Kamara, S., Prokhotskaya, V. Y., et al. (2006). Dissolved humic substances: ecological driving forces from the individual to the ecosystem level? Freshwater Biology 51: 1189–1210. Humic substances as mild stressors, antifungal effects, and ion uptake in Rio Negro fish.
- Humic substances. Part 1: Dissolved humic substances (HS) in aquaculture and ornamental fish breeding (2008). Environmental Science and Pollution Research 15(1): 17–22. doi:10.1065/espr2007.08.448. Stress, healing, and antifungal effects in fish culture.
- Nugroho, R. A., Manurung, H., Nur, F. M. & Prahastika, W. (2017). Terminalia catappa L. extract improves survival, hematological profile and resistance to Aeromonas hydrophila in Betta sp. Fisheries & Aquatic Life 25(2): 103–115.
- Yakubu, Y., Talba, A. M., Chong, C. M., Ismail, I. S. & Shaari, K. (2020). Effect of Terminalia catappa methanol leaf extract on nonspecific innate immune responses and disease resistance of red hybrid tilapia against Streptococcus agalactiae. Aquaculture Reports 18: 100555.
- Sensitivity of Terminalia catappa L. toward Aeromonas hydrophila (2020). Jurnal Perikanan dan Kelautan 25(2): 116–122. doi:10.31258/jpk.25.2.116-122.
- Priyadi, A., Lukman, Nur, B., et al. (2025). First Captive Breeding Report of Critically Endangered Wild Betta hendra. Aquatic Conservation: Marine and Freshwater Ecosystems 35(7): e70186. doi:10.1002/aqc.70186.