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Can You Put LEGO Bricks in a Fish Tank? The Answer

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Aquariums are a great hobby to get into and house some of the simplest, yet beautiful pets. They can be a great way to also express ourselves in our new pets tiny environment. However, there is always a certain amount of research that should be done when trying to decorate your fish tank. 

Legos can be put into aquariums with proper precautions. They should be sanitized properly, not crowd the tank, or have smaller holes in which fish can get stuck. With that being said, putting Lego blocks in an aquarium is not recommended by the Lego company, but you can use your own judgment.

You should think about this. In the rest of the article, I’ll briefly cover everything you should know to make an informed decision.

What are the cons of using Legos in aquariums?  

They can inhibit the growth of bacteria.

Just like other items that you may place in your fish tank, Legos can inhibit the growth of bacteria. This can lead to several problems with your aquarium that can potentially lead to the death of your fish. 

So what types of bacteria should you be careful of inhabiting your tank? Here’s a short list…

  • Aeromonas complex
  • Flexibacter, sp. 
  • Pseudomonas, sp.
  • Vibrio sp.
  • Mycobacteria, sp.

With that being said, it is crucial to upkeep your aquariums cleanliness. It is recommended to clean your tank about once a week, especially if you have multiple fish and decorations. 

If you only have a few with a larger tank, you could get away with cleaning your tank once every other week. When cleaning your tank make sure you use the proper solutions such as PH balancer, and a chlorine remover. 

lego figure inside aquarium

Fish can choke on small pieces. 

While most Lego pieces are of decent size, it is important to not put super small Legos in your tank. A fish could potentially think it is a piece of food and swallow it, leading the fish to choke. You also will need to ensure that all pieces you do put together are secured firmly. 

What are the pros of using Legos in aquariums?

If you’re looking for a unique way to add some personality to your fish tank, adding Legos to your tank is a way to do it. Not only are Legos a fun way to keep your fish entertained, but their unique design can also provide a benefit to your fish.

Interesting Environment that is Easily Changeable 

Legos can help to keep your fish healthy by providing an interesting environment in which they can explore and play. This gives them mental stimulation so they won’t get bored of their habitat. You can even change your setup each time you give your aquarium a cleaning.

Easy Cleaning 

Legos can be cleaned with warm water and vinegar. This makes for easy cleaning compared to other materials that can be used in fish tanks.

Legos can also help to keep your tank clean – as the pieces fall off the Lego blocks, they become trapped in the water and can be easily removed by the filter. So if you’re looking for a fun way to add personality to your fish tank, and you’re in the market for a unique design, give Legos a try!

How do you weigh down Legos in a fish tank?

The fact of the matter is that Legos naturally float because of the air inside of them. However, there are a few things you can do to combat this issue and make sure the Legos stay secure in your fish tank! 

  1. Metal Washers. You can attach metal washers on the inside of your masterpiece with a fishing line. This will weigh down the legos and make it safe for the fish.
  2. Large rocks. Some people will put large rocks on the inside of the Lego masterpiece to decrease the air trapped.
  3. Create the Lego masterpiece inside of the tank. Instead of creating the decoration outside of the tank, fill your aquarium with water and build the Lego one by one inside of the tank. This will decrease the air flow and make it possible for your Legos to sink. 

How much is too much decoration for your tank? 

Tank Size

When it comes to fish tanks, size really does matter. That’s why it’s important to know the dimensions of the different types of fish tanks.

For starters, the 10 gallon tank is perfect for small fish like goldfish, guppies, and dwarf gouramis. These fish can easily get along with other small fish, and they won’t require as much space as a 25 or 50 gallon tank.

The 25 gallon tank is great for medium-sized fish like cichlids. These fish can live together peacefully, but they’ll also need plenty of room to swim and play.

The 50 gallon tank is perfect for larger fish like barbs and tetras. These fish need plenty of room to swim and hide, and they’ll also need a place to lay their eggs. The tank and those larger, can also be used to house a multitude of small fish as well.

The 75 gallon tank is perfect for large fish like sharks and lionfish. These fish need plenty of space to swim and dive, as well as a place to lay their eggs.

The 100 gallon tank is perfect for nearly any type of fish you can think of. It’s large enough for most fish, and it has enough room to swim and lay eggs.

Room is Needed

Now that you know which tank size would best suit your needs, you will need to determine how many decoration is too much. Your fish will need room to swim and thrive without too many items in their way. 

It is typical to have one fish per gallon depending on the size. So let’s say you have 10 guppies (very small fish). You should have at least a 10 gallon tank for them to swim in. With that being said, you should have the next size up to add decorations, and plants. This ensures there is enough room for the fish to swim, gives room for decorations to hide in, and lay eggs. 

What materials are safe for aquariums?

When it comes to fish tanks, there are a variety of materials that can be used. Some of the most common materials include plastic, rocks, and wood. It’s important to choose the right material for your fish tank, as each has its own advantages and disadvantages.


Plastic is the most popular material for fish tanks, and for good reason. It’s versatile and durable, making it a great choice for those who want your decorations to last longer. There are also plastic tanks that come in a variety of colors and styles, so you can find one that fits your unique personality and home décor.


Rocks are another popular choice for fish tanks. They provide a natural habitat for fish, and they also look great in a tank. Rocks can be purchased in various shapes and sizes, so you can find one that matches the style of your tank perfectly.


Wood is a classic choice for fish tanks, and for good reason. It’s beautiful, rustic, and natural-looking, and it also provides excellent habitat for fish. You can purchase chorro logs to put in your tank, which provides a place for your fish to hide as well. 

Final Thoughts 

Needless to say, there are both pros and cons to using Legos in a fish tank. While the Legos manufacturer does not recommend using their products in aquariums, it has been done plenty of times with no issues. Just ensure the Legos you are using are clean, properly secured, and not too small!