17 Dangerous Things You Shouldn’t Put In Your Fish Tank

There are essential items every fish tank needs. 

But there are also dangerous things you shouldn’t put in your fish tank, some of which are sold in fish stores! 

As a veterinarian and pet lover, I am committed to helping you create the perfect tank to keep your fish healthy.

things you should not put in your fish tank

#1 Plastic

We have all seen colorful plastic decorations in an aquarium. 

And there are always cute plastic toys for sale at fish shops. But not all plastic is safe to put inside your fish tank.

Plastic contains many chemicals. 

When plastic is exposed to water for long periods, thousands of chemicals can leach into the water. Some of these are toxic and can harm your aquatic pets.

The only plastics which may be safe for your fish are food-safe plastics

You will be able to recognize these by their food-grade plastic symbol. 

But since plastic can release toxic chemicals if exposed to heat or water for a long time, we recommend avoiding all plastic if possible. 

#2 Ceramics

Ceramics may contain harmful chemicals. 

The glaze used to cover ceramic may contain lead, which is toxic to humans and animals. 

Water will eventually break down this glaze, which could lead to deadly lead poisoning in your fish.

As a basic rule of thumb, if a ceramic is not safe to eat, it is not safe for your fish. 

If a ceramic is labeled “Not for Food Use–May Poison Food,” do not put it in your tank. 

Ceramics labeled as dinnerware safe are less likely to break down and release harmful chemicals. Regardless, we recommend steering clear of ceramics to keep your fish safe. 

#3 Untreated Wood

Adding wood is a popular decorative choice to mimic a fish’s natural habitat. 

Wood may also serve as a hiding place for your fish, thus reducing stress. However, not all wood is safe to put in your fish tank.

When exposed to water, wood can leach harmful chemicals into your aquarium. 

Therefore, any wood you put in your tank must be non-toxic. To avoid toxic chemicals, untreated wood may seem like an appealing choice. 

However, untreated wood decomposes much faster and leads to spikes in bacteria and fungus.

An example of dangerous untreated wood is driftwood from saltwater or brackish water. 

This type of driftwood which washes up on shore contains lots of salt and cannot be used in freshwater aquariums. Please also avoid wood from trees with harmful bark or sap.

Treated wood is preserved and more resistant to decomposition in water. 

However, even treated wood can leach toxic chemicals into the water. 

Therefore, choose treated wood bearing the BMP mark, which meets standards based on Best Management Practices for the Use of Treated Wood in Aquatic Environments.

#4 Metals

Metals oxidize (or corrode) and create rust, which is toxic to fish.

Heavy metals, in particular (e.g., mercury, lead, zinc, copper), are a definite no-no given their severe toxic effects on fish.

Some people claim titanium and stainless steel are safer metal options because they do not corrode as quickly. 

However, all metals, including titanium and stainless steel, will eventually corrode, diminish water quality, and threaten your fish. Thus, we recommend avoiding metal altogether.

#5 Non-Aquarium Safe Plants

Aquatic plants serve many vital functions for fish. 

For instance, they serve as a source of oxygen, food, and shelter from predators. But not all plants are safe to put in your fish tank.

There are two main types of plants: terrestrial plants and aquatic plants. 

In general, terrestrial plants should not be used in your fish tank since they are not designed to survive in an aquatic environment.

Some plants are toxic to fish.

It is impossible to cover every plant species that could harm your fish. 

Before introducing a plant into your fish tank, we recommend looking it up to check if it is toxic to fish.

#6 Water Outside Proper Ranges

There are many species of fish. 

They live in various climates and require different water temperatures based on their natural habitats. 

If you plan to build a community tank with different types of fish, make sure all tank mates share similar temperature requirements.

Fish are cold-blooded animals. 

Therefore, their body temperature depends on the temperature of their water. 

Many normal bodily functions can only take place at specific temperatures. 

If your tank’s water is too hot or cold compared to your fish’s natural habitat, they will develop health problems and may die.

Providing an appropriate water temperature is necessary to keep your fish healthy. 

To maintain a consistent water temperature, you will need a submersible heater like this Submersible Aquarium Heater and an accurate aquarium thermometer like this Aquarium Thermometer on Amazon.

#7 Beach Sand

Most of us commonly associate fish with sand. But we must remember only marine fish have beach sand in their natural habitat.

It may come as a surprise, but beach sand contains potentially harmful substances. Unfortunately, beach sand often contains plastic and chemicals because of ocean pollution. 

Beach sand may also carry infectious bacteria, viruses, parasites, and fungi. Therefore, beach sand should never be used in your aquarium.

Including a substrate, like sand, in your aquarium has its benefits. Sand can improve enrichment by encouraging natural behavior in your fish. 

However, aquarium sand can store dangerous bacteria and lead to higher ammonia and nitrate levels. If you incorporate sand in your tank, ensure it is not untreated beach sand.

#8 Shells, Corals, Rocks

Due to their high salt content, shells, corals, and rocks should never be used in a freshwater tank. 

But if you have a saltwater tank, you may be inclined to simulate a marine reef environment by adding these substrates.

These are desirable substrates because they help filter harmful things, from ammonia to nitrites.

But these substrates are often harvested from the ocean, contributing to coral reef destruction. 

Furthermore, much like beach sand, if these substrates are taken from the ocean, they are likely contaminated with harmful chemicals from pollution.

Shells, corals, and reef rock contain calcium carbonate. 

If put into your aquarium, these substrates will react with the water, leaching calcium and altering the water’s pH.

For these reasons, refrain from using these substrate types for your tank.

#9 Degradable Products

Degradable products will break down, releasing potentially hazardous chemicals into the water. 

Products that degrade can worsen water quality and make your fish sick or even kill them. 

Such examples include things we have already discussed, such as plastic, ceramics, and paint.

#10 Chemicals

Chemicals should never be added to your fish tank unless labeled safe for fish and aquarium use. 

If a product is labeled for aquarium use, it is crucial to follow the instructions exactly.

Using a chemical not explicitly designed for fish tanks may be deadly for your aquatic pets. For example, cleaning products like bleach, chlorine, and detergent are toxic to fish. 

Even cleaning your fish tank with an inappropriate product could be fatal for your fish.

Look for warning labels indicating a product is toxic to fish. 

Examples include “this pesticide is toxic to fish,” “toxic to aquatic environment,” or the symbol of a fish and tree in a diamond border.

#11 Anything Painted

Although painted decorations may look pretty, the paint will cause ugly problems for your fish.

Water-based paints break down in the water. 

As paint breaks down, it may peel, and your fish may mistake the lifting paint for fish food. 

The resulting paint particles will end up in your fish’s digestive system and gills if eaten.

Paint is made up of compounds similar to plastic. 

But when paint breaks down, the chemicals released are even more toxic than plastic. 

So while colorful aquarium decorations may be tempting, it is best to avoid anything painted. 

#12 Wrong Foods

Diet is highly variable depending on the species of fish

Fish are either herbivores, carnivores, or omnivores. 

So it is important to do your research regarding the best diet for your species of fish.

While fish food comes in different forms (e.g., dry, frozen, live), only feed food explicitly designed for fish. 

Do not feed human food or diets intended for other types of pets, as these may be toxic to fish.

Although not toxic, just a friendly reminder: excess fish food breaks down over time. 

Extra food will decompose and create more waste. 

Overfeeding also leads to higher ammonia levels which can affect your fish’s health. Strive to feed enough food to keep your fish at a healthy weight without making them obese. 

A helpful tip is removing uneaten food after several minutes of feeding.

#13 Your Hands

If possible, we avoid regularly putting our hands in our fish tank.

Dirty hands can introduce infectious diseases like bacteria into the aquarium. 

Additionally, products like hand sanitizer and lotion should not be introduced into your fish tank. 

So if you need to put your hands in the tank for any reason, clean them first.

Washing our hands with soap and water once is not appropriate. This is because soap residue may harm your fish. 

The best way to clean your hands is to wash them once with soap and water. Then, wash your hands at least one more time with water alone.

Please remember, if you have broken skin on your hands, they should not be exposed to aquarium water. 

If, for some reason, you must put your wounded hands in the tank, wear aquarium-safe gloves to lower your chance of infection.

Avoid touching your fish if you must put your hands in your fish tank. 

Fish have a unique protective layer of slime over their scales. We do not want to rub this layer off or injure our fish.

Human hands are not part of a fish’s natural environment. 

Your hands may scare and stress them out. But, conversely, aggressive fish may nibble at your hands if they view them as a threat.

There will be unavoidable times when you must put your hands into your aquarium. 

The main things are to do so as infrequently as possible, clean your hands appropriately, and avoid touching your fish.

#14 Sharp Objects

You may have pet-proofed or child-proofed your home before. In the same way, it is important to ensure your fish’s environment is as safe as possible.

Take time to check for and remove sharp objects that could hurt your fish. Also, inspect the tank and anything you plan to put inside it.

Sharp objects can cut your fish and cause wounds, which could become infected.

#15 Dry Ice

Dry ice may sound appealing for Halloween parties or create a romantic atmosphere for your wedding dance. 

But if you are thinking about reaching for dry ice for your fish tank, think again.

Dry ice is the solid form of carbon dioxide. Putting dry ice in your aquarium may cause frostbite, suffocation, or an explosion! 

Keep your fish and your family safe by keeping dry ice out of the equation.

#16 Large Fish Breeds

Before buying fish for your tank, research how big a species will grow. 

Some species of fish will get too large for hobby-sized aquariums to accommodate. 

For example, Arowanas and certain catfish require ponds or public aquariums once fully grown.

Keeping inappropriately large fish species in a tiny enclosure is unethical and may kill them. 

Furthermore, housing multiple larger fish increases the risk of overcrowding, leading to mobility and behavior issues.

Larger fish may also be more likely to eat other fish. 

Thus, keeping large fish in a community tank setting could endanger the other, smaller species in your aquarium.

Our post on 40 unique freshwater fish pets gives you a lot of information on popular fish and the sizes they mature at to help you pair your fish.

#17 Other Animal Types

Fish have unique environmental requirements which are very different from other animals. 

Most notably, they require water. Additionally, their environment needs to be within a specific temperature range.

Other types of animals, including reptiles, often have environmental needs which do not match those of fish. 

Thus, creating a single environment appropriate for all animals in your enclosure would be impossible. 

This will lead to poor quality of life, health problems, and even death.

Another concern would be predation. Other animals, like turtles, may eat your fish. 

Therefore, we recommend sticking to fish and keeping all other animals out of your tank. 

Takeaways About No-Nos for Your Fish Tank

Whether you are a first-time fish owner or an experienced fish lover, some of these potentially harmful items may have surprised you. 

There is plenty of data about toxic items to avoid in dogs and cats. 

But as the information about more exotic pets, like fish, continues to grow, it is okay to be cautious if something seems fishy. 

Help educate your fellow fish owners by sharing this article!

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Dr. Amanda Takiguchi is a veterinarian working to promote client education through her writing. She primarily works with dogs and cats but also loves exotic species like betta fish! When she is not promoting animal welfare, Dr. Takiguchi loves to dance and spend time with her two dogs.

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