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Are Legos Safe for Aquariums?

In my house, Legos can be found in numerous locations and in more shapes and styles than my son has time to play with them! For some reason, just last night, I was thinking I should put some of his unused Legos together and place them in my fish tank. We have a 10-gallon aquarium in our family room that would look great with some Lego toys in it. However, since Lego pieces are plastic, I knew I had to do a bit of research first.

Are Legos safe for an aquarium? Yes, Legos are safe and can be placed in an aquarium. Lego pieces are very hard inert pieces of plastic, meaning Legos are chemically inactive and will not leach toxins into a fish tank. Sanitizing the Lego pieces beforehand is recommended. 

That’s all I needed to read. So now, I need to select which pieces get to enjoy being submersed in my 10-gallon aquarium. One thing that came to mind though is what about small pieces and larger fish, or will small pieces be knocked off the fish and what could I do to prevent this. Well, I just happen to know what to do and will share that with you below.


Is it Okay to Put Legos in a Fish Tank?

Yes, you may place sanitized Legos into a fish tank without worrying if toxins will be released into the water. But what about the smaller pieces that might fit into a fish’s mouth and choke it?

My suggestion is to glue all the Lego pieces together with Super Glue or any other safe for fish glue.

I won’t pretend I know how to say this word but the main ingredient in Super Glue is called Cyanoacrylate, which is stated as a safe for fish ingredient. Apparently when exposed to water Super Glue cures much faster than without. This makes sense now because when I get it on my fingers and try to rinse it off, my hands feel like a vice of glue is tightening on them.

**I found this really cool Lego kit on Amazon. How would your fish do with this big scary shark? **

Can You Put Toys in a Fish Tank?

You can most likely find many toys at your local fish store that are aquarium safe. For toys you find around your house, I would recommend you do a bit of research on the types of plastic or other materials to make sure it will not leach anything into the water.

This could obviously be tragic if you just stuck anything in your fish tank without doing a bit of research.  As previously mentioned, Legos for example, are a hardened inert plastic meaning they will not leach anything into the aquariums water.


How Do You Sanitize Legos Before Placing into Your Tank?

Since you plan on putting the Legos into your fish tank, I would suggest you use a product that will not harm your fish just in case some residue is left on the Lego pieces. Vinegar is your friend for this cleaning task.

I would mix a few tablespoons of vinegar per liter of water and use that to clean them of. Try using an unused toothbrush if you need to give them a good scrubbing.


How to Ensure Lego Pieces Don’t Float in Fish Tank

We kind of touched on this already when talking about gluing everything together. You could even glue the Legos to a Lego base plate and bury the plate in the substrate if you don’t want it visible. Gluing Lego pieces to one another and to other objects in the tank will help greatly in keeping everything in place right where you want everything to be situated.

This is where your creative and problem-solving side needs to shine. There is no right or wrong way to do this. Just make sure whatever you end up with works well with the tank as a whole. You want the fish to be safe and the water to be able to flow and circulate as much as possible.


Are Legos Water-Tight?

From what I can tell, Legos appear to be watertight. There is no indication to think otherwise.

The only air issue you might have is when first placing the Legos into the water there could be air pockets inside figures etc. Just turn the pieces upside down and the air should float out.


What Plastic is Aquarium Safe?

Whether you want to place Legos or other plastic pieces into your tank, there are some plastics to look for that will be safe. I know because I just researched it and am happy to report it here.

Plastics that are made with High-Density-Polyethylene are chemically stable and inert. They will not leach toxins into your fish tank.

On any piece of plastic look for the following markings. HDPE stands for High-Density-Polyethylene and is much easier to stick on a toy than the full wording. As well, look for the numbers 1 to 7 as these are HDPE. The safest is numbers 5 then 2 then the rest. These ratings are based on human consumption however this can be extrapolated into the safety of using these plastics in your fish tank.



We learned today that Legos are safe to place into your aquarium. As well, before you do so, you should consider sanitizing the pieces to ensure you aren’t putting something into the water that will affect the fish negatively. If you need to glue pieces together, it is safe to go ahead and use some Super Glue for this. You can also find aquarium glues and sealers at your local fish store.

Lastly, keep a close eye on your fish for the weeks following placing something in their habitat that wouldn’t normally be there to ensure they are looking healthy and to see that they have received the object with open um fins so to speak. If you find the fish are steering clear of your Legos, then maybe you need to take them out. We want our pets to happy.

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