Can Plants Kill Fish: Considerations and Basic Precaution

We all know that animals, aquatic and terrestrial, can harm plants. Usually by eating them, but they can also uproot them and break them.

But are plants really that helpless and incapable of inflicting damage?

Can plants kill fish and other animals in your fish tank?

Plants, for the most part, just look cool and chill out there, but they are living organisms that inherently want to grow as much as possible. Given the right conditions, they will cover the Earth. Ask wheat.

Aquatic plants can harm fish and other animals. If plants grow too densely species with large fins like bettas can get entangled. Also, plant dead matter decays and produces ammonia. At night, plants don’t produce but consume oxygen, which could be hazardous without an oxygen pump.

Hard, pointy artificial plants could also damage fish.

It is important to understand that these dangers are mostly serious considerations than actual threats. Plants, in general, are hugely beneficial for any ecosystem, but in the confines of an aquarium, they can get out of hand. Actually, that’s true for open-water habitats as well. Aggressive species like water hyacinth have destroyed numerous aquatic ecosystems across the globe throughout the years.

Let’s check out in detail whether and how plants could become dangerous for an enclosed water ecosystem.

Dense Vegetation Can Be Harmful to Fish

Dense vegetation is great, but plants must be trimmed and maintained. If left unattended for too long, stem plants in particular will grow quickly and thickly. A thick underwater jungle poses a certain danger to many species. I’ve seen reports that certain terrestrial plants can also be harmful, but I don’t have experience with them to confirm either way.

However, fish with large, gentle fins like bettas could get entangled if they forage for food in dense shrubbery. This could lead to fin damage or even death if you aren’t there to save the fish. Large angelfish could be entrapped by a forest that breaches the surface.

Admittedly, such scenarios are unlikely, but you must consider the much more real and practical impact live plants have.

They take up space.

Too many plants, even if they don’t entrap hapless beautiful fish, will eat up swimming space. If you have active swimmers like danios, clogging the space with plants will restrict their free movement.

Additionally, loads of plants are bound to limit light penetration. Bottom-dwellers might be able to make their way among the plant stalks more freely, but their colors could start fading. If they don’t receive enough light, the fish will adapt to their darker environment, with varying success.

Could the light shortage in places lead to algae? It is not out of question, even though it will find it hard to procure enough nutrients in a heavily planted tank.

Can Plants Suffocate Fish?

Plants are notorious for producing oxygen. One of their main sources of energy is CO2. It is essential for photosynthesis and one of its byproducts is oxygen.

The thing is that plants don’t photosynthesize 24/7. Even if the lights are on all the time — not recommended if you were wondering — plants won’t constantly feed on CO2. They need time to rest, just like any living being. When they aren’t actively producing oxygen, plants consume it. In fact, they consume it all the time, as they breathe.

So, when the plants are not photosynthesizing, they are simply consuming O2, like the rest of their tank mates.

A heavily planted tank could experience a shortage of oxygen in the hours of the night. If the majority of the surface is covered in plants like duckweed, and there is no additional oxygen input the tank could experience a quick buildup of gases. Small water surface limits the free exchange of gases and could lead to sudden crashes.

Is duckweed the culprit, in such situations? If left unattended, it will happily cover 100% of the surface, condemning all plants below to a gloomy death. What’s more, it will benefit massively from the impeding decay, gorging itself into layers of duckweed.

It will call itself Winweed.

But as far as suffocation due to oxygen shortage is concerned, that’s not a realistic danger.

Are Decaying Plants Dangerous?

Thick vegetation makes it harder to clean your tank and also produces more dead leaves.

Across the Web, you will hear the advice to remove dying leaves in order to avoid ammonia spikes. It is true enough that decaying organic matter — plants, dead fish — produces ammonia. But how much and how quickly does it produce it?

Dead fish produces much more and so does uneaten fish food. Dead or dying leaves contribute, but they are also a food source. Some species like Gouramis and barbs like to nibble on decaying leaves. Microorganisms like helpful bacteria and simple invertebrates also use them as a main food source.

So, decaying leaves could contribute the higher ammonia levels, but they have to be dying in throngs, and quickly, If this is happening, the ammonia released in the process would be a minor problem, as something else would have gone horribly wrong to cause such mass death.

Should you remove decaying leaves?

If they aren’t too many — and healthy plants will shed leaves every now and again — you may very well leave them be. Especially if the shed leaves are at the surface, as most of their decaying impact will be happening outside the water column.

That’s particularly true for larger, settled tanks. In a new aquarium under 20 gallons, I’d recommend removing decaying leaves right away to avoid fluctuations in the water chemistry. Once six months have passed and you see no major spikes in ammonia between maintenance sessions, a couple of rotting leaves can make a few fish and shrimp happy without affecting the rest in the slightest.

Are Fertilizers Dangerous for Fish?

Overall, fertilizers are safe for fish.

But there are a few things to consider.

First off, if you are going to use liquid fertilizers, always go for products made for water plants. Many fertilizers designed for terrestrial plants contain ammonia that can destroy your livestock.

Second, use the recommended dose and do so regularly. Now, practically all commercially sold fertilizers are very safe and even a large overdose is unlikely to harm your fish or shrimps (even though shrimps often are more sensitive to fertilizers).

But too large a dose is likely to ignite an algae bloom and that’s no fun. Keep up with regular, within-dose fertilization and your plants will thrive, while your fish won’t suffer.

Don’t overdose, even if you’ve missed the regular fertilization time.

A safe middle ground is to use root tabs, but they work mostly for root-feeding plants.

CO2 dosing can lower the pH of your water column rapidly, so it is best to use a steady inflow of the gas. Sudden changes in pH (or temperature, for that matter) can send your fish reeling.

Can Aqua Soil Damage My Fish?

Enriched substrates won’t harm your fish directly. However, they require frequent water changes, especially in new tanks, to keep the water chemistry balanced and to prevent algae from spawning.

In general, aqua soil is safe for your aquatic pets, but I wouldn’t recommend it for beginners. It does help plant growth massively, which could lead to overly dense vegetation if you cannot maintain your tank regularly.

The real issue is algae, though. It is hardly harmful to fish, but it ruins aesthetics and plants big time.

Can Plastic Plants Hurt Fish?

Most plastic plants will not hurt your fish. I’ve seen anecdotal reports that sharp, pointy, grass-like leaves can harm fish by either tearing their fins or poking their eyes out.

I admit, I have had very limited experience with fake plants, ages ago when I started my first aquarium. I didn’t like how they looked (fake and unnatural) and quickly got me some live plants to put in my gravel.

All the same, I’d avoid putting cheap fake plants. Besides being poorly made, they could be crafted from toxic materials. You better spend a couple of dollars more to get better quality plastics or even silk plants. There are very decent replicas out there that look great and will certainly not harm any of your aquatic pets.

Now, live plants have many benefits, but artificial ones also have some merits which I am eager to explore. They are great for plant-eating fish setups and require less maintenance.