Just because you can, doesn’t always mean you should…
Putting LEGO® in a fish tank is an interesting form of LEGO® aesthetic. The little colorful bricks, combined with the serene beauty of an aquarium, does seem alluring.
But should you put ABS bricks in water?
You should take certain precautions to ensure the safety and well-being of your fishy friends. Not just for any chemical seeping, but to ensure none of your fish eat the lil’ LEGO® parts.
Let’s explore the dos and don’ts of using LEGO® decorations in a fish tank to see if this is something you should actually do!
They Can Be Used As Decorations For Fish Tanks, But You Have To Know What You Are Doing
Absolutely, you can use LEGO® bricks as decorations in a fish tank, but it’s crucial to follow some guidelines to create a safe and comfortable environment for your fish. First and foremost, make sure to sanitize the LEGO® pieces thoroughly before introducing them to the tank. At the very least, this will reduce any possible contaminants in the tank.
The Chemistry Behind LEGO® Placement In A Fish Tank
LEGO® is made of a highly durable plastic that is insanely strong called ABS, or acrylonitrile butadiene styrene. By nature, this polymer does not dissolve easily in water. A 2020 study by the University of Plymouth found that LEGO® bricks can persist in marine environments for 100 to 1,300 years, depending on the brick’s composition and weathering conditions. Salt water is more corrosive, so it can potentially crack the bricks over extended exposure. However, in a controlled freshwater aquarium, ABS plastic should stand the test of time.
Don’t Overcrowd Your Fish’s Space With LEGO®
When arranging the LEGO® bricks in the tank, ensure they don’t overcrowd the space. Fish need room to swim freely and explore. Overcrowded fish tanks can lead to stress, health issues, and create an environment where pathogens thrive.
You Don’t Want Fish To Get Caught In Any LEGO® Openings
Another essential consideration is the size of the fish and the openings in the smaller LEGO® pieces. Smaller fish could get stuck or trapped in the holes, leading to injury or distress. Make sure any LEGO® structure you introduce into the tank is large enough for all fish to fit through!
LEGO® Does Not Endorse The Use Of LEGO® In Fish Tanks
For your information, the LEGO® company does not endorse using pieces for this. It does not mean you can’t do it, but it should make you question why/if you should go through with it. Perhaps you can place the LEGO® pieces in a fish-tank-safe acrylic box within the tank?
Consider The LEGO® Icons Tropical Aquarium Instead
If you love both LEGO® and aquariums, there’s actually a better solution that launched in November 2025. The LEGO® Icons Tropical Aquarium (10366) is a stunning 4,154-piece set that lets you build a detailed brick-built fish tank complete with four model fish, coral, seaworms, an oyster with a pearl, and even a hidden treasure chest. Rotating dials and cranks animate the swimming fish and swaying coral. It measures over 20.5 inches wide and makes for an incredible display piece that requires zero water changes, no fish feeding, and absolutely no risk to any aquatic life. Sometimes the best aquarium is the one you build yourself — out of bricks!
It Can Be Done, But Read This Guide And Do It Safely
Incorporating LEGO® decorations could be a stellar way to make your existing tank more beautiful and fun for your under sea friends. However, make sure to do ample research and ensure your tank can support having LEGO® in it. Also, please invest in an incredible filter for your aquarium friends.
Lastly, overall, while it is possible, I would try to recommend keeping LEGO® out of your fish tank! LEGO® are dry weather friends — happy building!
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