10 Things You Must Never Put in a Freshwater Fish Tank

10 Things You Must Never Put in a Freshwater Fish Tank

Adding a freshwater aquarium to your home is a great option, as it can be a great décor piece, a fun pastime, and an excellent first pet for children. While fish are relatively easy to care for, you do want to start them out with the best tank possible. There are several products available today that are marketed as aquarium safe but may not be the best choice for your freshwater tank.

Are there things you must never put in your freshwater fish tank? Yes, here are some items you must keep out of your freshwater tank, from those that are just best avoided, to those that can cause death or illness to the fish. The top 10 items to leave out of your freshwater fish tank are:

  • Plastics
  • Wood
  • Beach sand
  • Shells or coral
  • Sharp objects
  • Coins
  • Electronics
  • Certain plants
  • Types of fish
  • Your own hands

In addition to discussing what you should not put into the tank, we will also discuss ways that you can ensure your tank is as safe as possible for your fish. You want to choose the right water, rocks, plants, and other decorations for your tank. Creating a safe, healthy atmosphere guarantees your fish have a longer and healthier life in your home.

What Items Should Not Go into a Freshwater Tank?

While the items on our list may not immediately kill or harm the fish, they are not recommended by most fish researchers. These items may give off harmful chemicals, too closely resemble food causing choking risks, or cause bodily harm to the fish.

Some commonly used items that are actually dangerous to your fish and should be avoided are:

#1 – Plastic Decorations or Products

10 Things You Must Never Put in a Freshwater Fish Tank Plastic Decorations
Picture: Plastic Decorations

Many pet stores and aquarium stores actually sell products made from plastic, and these are commonly bought. In fact, a lot of fish owners do not realize the harm that plastic can do because they feel safe buying from well-known sources.

However, after a while in the water, the plastic can begin to release toxic chemicals, and you may not know this until it is too late.

The only plastic products that may be safely used are those that are food-grade safe, which will be marked with a triangle made from three arrows. This sticker is usually found on the bottom of the product, and you should search for any plastic items you are considering use such as Fish Tank Decoration from this marker.

If you are unsure if the plastic items you are considering are safe, it is best to avoid them.
Also, if the plastic decorations you are finding in your local store have paint that has not been sealed, they can release even more toxins into the water.

If you feel the absolute need to purchase a plastic decoration, make sure it is food grade safe, and any paint is completely sealed. The best bet is to avoid plastic altogether.

#2 – Untreated / Unsafe Wood

10 Things You Must Never Put in a Freshwater Fish Tank - 10 Things You Must Never Put in a Freshwater Fish Tank Untreated Wood Fsihkeepup.com
Picture: Untreated Wood

If you have been looking at examples of freshwater tanks, you have probably noticed that most contain some type of wood, which can make this suggestion a little confusing. While wood itself should not be avoided and can work well in most aquariums, you must be cautious of the type of wood that you use. Freshwater aquariums must use treated wood to avoid harming fish.
You should avoid using any untreated wood and finding your own wood in the outdoor areas near your home. Instead, visit a local pet store and find quality treated wood that matches your aquarium and is safe.

Even if you find a nice branch that you think will work perfectly, it is best to simply leave it there.
If you are searching for certain types of wood to put in your tank, some great options are driftwood and hardwood. These take longer to decompose and do not have as drastic of an impact on the water chemistry. However, any type of quality treated wood should work.

#3 – Do Not Add Beach Sand

10 Things You Must Never Put in a Freshwater Fish Tank - Sand Beach Fishkeepup.com
Picture: Beach Sand

Beach sand is another option that you should not use in its natural form, as it has not been treated and can be harmful. Beach sand in nature is usually polluted and can contain chemical residue, which is harmful to your tank environment.

While it may seem to make sense that fish can handle beach sand, since the ocean has fish, the environment is drastically different. The ocean is constantly renewing, but a small aquarium cannot handle these chemicals.

While sand itself is a great addition to many tanks, you always want to get it from a trusted source. Even if you think beach sand looks amazing, it is hard to find beach sand that has been treated and comes from a great supplier. The search to find safe beach sand may not be worth the time and energy you put into it.

#4 – No Shells or Corals

10 Things You Must Never Put in a Freshwater Fish Tank - Shell Fishkeepup.com

Coral and shells will naturally add calcium to the water. While this can be great for certain fish species, it is not ideal for freshwater fish. They do not need this added calcium, and using shells or coral is generally advised against.

Seashells can actually modify the PH hardness of the water in your aquarium. This can cause difficulties for the fish and can also make it harder for you to maintain the tank and the tank environment. If something can cause a chemical change to the environment of the tank, you should not put it inside the tank.

#5 – Check for Sharp Objects

10 Things You Must Never Put in a Freshwater Fish Tank - Sharp Objects Fishkeepup.com
Picture: Sharp Objects in Fish Tank

You should avoid using any decorations that may have sharp edges or cause a physical threat to the fish themselves. Some decorations you should never use are glass, painted glass, or decorations that may be sharp or easily broken.

Even if a decoration seems safe, you will want to run around the edges and look over it well to guarantee that there is no area that could hurt your fish.
If your fish does accidentally get cut by a decoration or other item in the tank, it can easily get infected and is difficult to treat.

This often leads to the death of the fish. To avoid this, you also want to frequently check the objects you have in your tank for any breaks or other issues. If anything is not intact or seems that I could harm the fish, you want to remove it immediately.

#6 – Avoid Using Coins – and Any Other Copper

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Picture: Coins Photo courtesy: Flickr CC 2.0

Almost everyone has seen a large fountain or tank at a museum or park that has fish, yet still has people throwing coins inside. While this may not directly harm those fish, you should never try to do this at home or in smaller aquariums.

Unlike larger fountains, you are not constantly cleaning your aquarium, it is much smaller, and you simply should never allow anyone to throw coins inside. Coins are made from copper, and this can be very dangerous for fish, as the metal releases bad toxins, and the chemical can cause injuries.

Fish and other aquatic species cannot handle copper toxins, and it will quickly become life-threatening. The best way to protect your fish is to never place a coin or other metals inside the tank.

#7 – Don’t Try to Add Electronics

10 Things You Must Never Put in a Freshwater Fish Tank - Electronic Devices Fishkeepup.com
Picture: Additional Unsafe Electronic Devices

Most tanks come with some form of lighting that you can use to illuminate the tank and better see the fish. However, you should avoid adding any lighting to the bottom of the tank, especially near the water.

Often, it is best to simply purchase a tank that already has lighting on the top of the tank, and if you feel like adding lighting, it should only be on the ceiling of the tank.
In addition to avoiding adding your own lighting, you will also want to avoid adding any battery or sound devices in the aquarium.

These are hazardous and can cause shock to your pet fish. In addition to avoiding these electronics causing shock, they can also lead to rush particles from any batteries that they may have.
If your fish consume these rust particles or they are abundant in the water, it can also be life-threatening.

Essentially, it is not necessary to add any new sounds, lights, or other electronics to your pet fish. Overall, it is best to stick with tanks that already have lights or only add simple, easy to maintain lights on the top of the tank.

#8 – Check Any Plants You Add

10 Things You Must Never Put in a Freshwater Fish Tank - Fish Tank Plants Fishkeepup.com
Picture: Fish Tank Plants

If you do add any plants to your aquarium, you want to make sure they are fish safe. You should always avoid anything that may become thorny or sharp. These can easily prick or hurt the fish and other aquatic species you may be interested in adding.

While there are some safe plants for freshwater fish, you should always be careful about what you add. You will want to safeguard the fish against any sharp plants.

#9 – Add the Appropriate Fish Species

10 Things You Must Never Put in a Freshwater Fish Tank - Add Only Appropriate Fish Species Fishkeepup.com

If you are not as experienced when it comes to designing a freshwater tank, you will want to research the types of fish you would like and how well they work together. No matter how large of a tank you choose, you want to have fish that cooperate and do not fight each other.

You want to make sure that whatever breeds you choose can cohabitate well. Another important factor is that you choose fish that will fit well in the tank that you are using. If the fish you are choosing grow to very large sizes, you will want to avoid these fish. You want whatever fish you choose to fit inside the tank well and not grow too large for it.

#10 – Make Sure You Use Clean Hands

10 Things You Must Never Put in a Freshwater Fish Tank - Only Clean Hands Fishkeepup.com
Picture: Only Clean Hands

As a fish tank owner, there will be times that you have to occasionally put your hands into the tank. It is extremely important that your hands are not contaminated in any way. This means you should have clean hands that have no traces of soap, lotions, or creams present.

When you use hands that do have any form of chemical on them, they will leave a residue behind that can be extremely harmful to the fish. If you need to do any work on your fish tanks, you will want to make sure they are clean. After thoroughly washing your hands, you will want to rinse them extremely well and make sure no soap is left behind.

Tips for Creating the Best Freshwater Tank

Now that you know more about what to avoid when creating your freshwater tank, you need to learn more about what you can use and how to set up the best tank possible. There are a lot of things that you will need to put into consideration when making your ideal tank.

We will jump into some things that you should keep in mind when starting your freshwater aquarium.

Not All Waters Are Created Equally

While this may seem self-explanatory, there are certain types of water that work better for freshwater aquariums than others. Some of the water types you may choose from are:

Tap water:

This is the easiest type of water to use because it is readily abundant for most tank owners and can be used from your home. Tap water is a cheap source, you simply need to add a water conditioner to the water to ensure that it is ready for your fish. Water conditioner is used to remove any chloramine from the water, which is commonly found in most tap water. To remove chlorine, you can always let the water sit for a day or two. How to making Tap Water Safe – read more here.

Well water:

if you still have a home that has access to well water, this is another good choice for your aquarium and can easily be used. You should always pay attention to the hardness of your water, because well water often has dissolved minerals. These minerals can make the water hard, but some fish species can adapt to this quickly. However, some fish cannot adapt and well water is not recommended.

Lake or river water:

While this can be used, it is not recommended, and most fish enthusiasts actually advise against it. When you use lake or river water, it can risk adding industrial pollution to your tank and fish parasites. Essentially, this water is too wild for use in your aquarium and is not the best source.

Reverse osmosis (RO) water:

If you are unfamiliar with reverse osmosis water, this is essentially pure h20 because it has been processed to get rid of all substances. This is one of the more difficult types of water to use because it is harder to find, but many fish stores do sell it. In some cases, this means that a change in pH cannot be handled well because there is no minerals acting as a buffer to the water.

Bottled water:

If you do not want to use your tap water, bottled water is always a great option. It can cost a little more to fill up your aquarium fully with bottled water, but it is still relatively inexpensive. The main thing to look into is that the only ingredient on the bottle is water, and there are no additives that can be deadly to the fish.


This is another type of water you should simply avoid because it is not worth the risks. While rainwater may seem natural, it can actually contain much more pollution than you think, even outside of the city. For most fish, rainwater is not the best option.

Choose the Best Aquarium Rocks

Similar to shells and coral, using certain rocks can actually increase the calcium levels in your tank. The best tanks have softer water with a low pH balance, which means using rocks that increase calcium levels is not ideal.

Essentially, these rocks will make the water harder and can actually harm the fish.
If you are hoping to use rocks in your fish tank, you will want to choose from options that sturdy but do not let off much calcium or salt. Some commonly used rocks that are safe are:

  • Lava rocks
  • Shale
  • Slate
  • Basalt Rocks
  • Quartz

Some rocks that have high salt or calcium content that you should avoid are:

Final Tips and Tricks for Happy Freshwater Fish

You should now have a good idea of what to use and what not to use when it comes to your freshwater aquarium. When setting up your tank, you may want to do some additional research into your specific fish to learn more about what they can tolerate and what they cannot be around. Some final tips and tricks you should keep in mind when it comes to creating the best freshwater aquarium possible are:

  • Only use decorations that are highly tested and authorized, guaranteeing that they are safe. Not all decorations sold on the market today are safe for regular use in a freshwater aquarium.
  • Never overcrowd your fish tank with too many fish. You should always limit the different types of fish that you use so that they do not interact negatively.
  • If you happen to lose a fish, you will want to remove the corpse as quickly as possible. Allowing a fish to decompose in the tank can spread infections.
  • Only give your fish high-quality foods that have been purchased from a reputable store; this will help with growth and keep fish healthy.
  • If you ever have any questions regarding your fish, the tank, or the overall health, you should ask an expert in your area for assistance.

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