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Fish Aquariums And LEGO – Is LEGO Safe For Your Fish?

Fish Aquariums And LEGO – Is LEGO Safe For Your Fish?

Whether you are considering putting some LEGOS into your fish aquarium or out of curiosity, you are wondering whether it is safe to put LEGO inside your aquarium. I had the same question, so I went online and did a bit of research and I decided to write this article to collect all my findings.

So, is LEGO safe for your fish? Yes, using the precautions we show in the rest of this article, LEGO is safe for your fish because it is made from ABS plastic. ABS is very stable and does not release any chemicals to the water so it will not be a hazard for your fish.

Keep reading to find out how to make LEGO safe for a fish aquarium

How to make LEGO safe for an aquarium

As we said, LEGO is safe to use in a fish aquarium. However, there are certain steps you need to follow in order to avoid safety hazards. The biggest hazards when placing LEGO in a fish aquarium are the following:

  • Floating LEGO pieces
  • Other hazardous material that is somehow stuck to your LEGO
  • Pieces that can directly damage your fish
  • Pieces that are not waterproof or are very difficult to clean

Let’s have a look at each specific one separately.

How to ensure that LEGO pieces will not float in the fish tank

The first main hazard that comes from placing LEGO inside your aquarium is floating pieces. As we also mentioned in our “Tips To Restoring Discolored LEGO”, LEGO pieces have the annoying habit of floating in water because of their density. These floating pieces can become a hazard for your fish and should be avoided at all costs.

Below we give you four different strategies for ensuring that your LEGO will not float in the fish aquarium.

Use LEGO boat weights

The first idea, which is also our favorite one, is to use LEGO boat weights. These are basically modified bricks that have an added weight to them. This added weight gives your LEGO set enough total weight so that the density will become higher than that of water so they will not float.

This is our favorite option because you only need to add a LEGO piece or a few LEGO pieces into your set to make it work. That means fewer problems with compatibility or with the additions damaging your LEGO set. It also keeps it nicer-looking because there are no weird extra pieces around.

In the following link that goes to Bricklink (the place to buy second-hand LEGO pieces), you can see an example of a LEGO boat weight.

Fill the LEGO construction with sand or gravel

The second idea is to fill your LEGO construction with sand or gravel. This requires much more work than the first idea and it comes with many more dangers. But, depending on the type of set you want to put inside, it is also a viable option.

It requires much more work because you need to decide where you are going to put the sand or gravel and also make sure that you do it in a way that it will stay there long term. What you don’t want is to do that and find after a few days or even hours that the sand is gone and your LEGO is floating around.

It comes with many more dangers because you can damage your LEGO when adding the sand or gravel. These are hard materials that can scratch your LEGO if you are not careful, so be careful when trying this.

Glue them to the aquarium or to other decoration elements

If you don’t want or can’t buy LEGO weights and you are not sure about adding sand or gravel, you can also consider gluing your LEGO to something else to keep it in place. If you are going to do that, though, there are a few things you need to keep in mind.

First, make sure that the glue you select is safe to use in water. There are many types of glue that have hazardous substances that will damage your fish and make placing your LEGO in your fish aquarium not safe. So, if you choose this option, just make sure that whatever type of glue you buy is safe to use inside your aquarium. The best option is to buy something specifically used for aquariums.

Second, think carefully about where you want to glue them.

If you glue them directly to the aquarium, bear in mind that you will not be able to easily take the LEGO out for maintenance. So, basically, you will need to keep your LEGO inside your fish aquarium for as long as you want it there. And once you decide to take it out, there is the risk of damaging the pieces while trying to take it out or simply having glue stuck in your LEGO pieces.

If you glue them on a decorative element, you could take out both the LEGO construction and the decorative element together for general maintenance. In that sense, this solution is better. However, you will have the same problem as before once you want to unglue it from the other element.

Anchor them to other elements

A less intrusive option than the glue is to simply anchor them to other elements instead of gluing them. This method has advantages and disadvantages compared to gluing.

As an advantage, it is much easier to disconnect the lego set from the anchoring for general maintenance or if you want to remove it completely. The level of difficulty will vary depending on the type of anchoring you use, but in general, it is much less intrusive than gluing.

As for the disadvantages, there are mainly two. First, it is much more complex to set it up. Usually, there will be many more parts in the anchoring system than simply gluing something, so it will be much trickier to set it up correctly.

Second, you also need to consider how the anchoring system will affect your fish. With the glue, as long as you choose a safe glue, you are good to go. Anchoring is a different story. Is the anchor safe? Will fish get somehow trapped in it or can it damage the fish in any way? Is it sturdy enough or will your LEGO float away after a few hours or days?

How to sanitize LEGO before placing them into your tank

Now that the fixing part is clear, let’s look at the second hazard, or how to make sure your LEGO pieces do not have any other hazardous materials that are somehow attached to it.

There are a lot of different ways to sanitize LEGO and we have covered it all in our post “This Is The Best Way To Clean LEGO”.

For this task, we would recommend cleaning following the advice from LEGO, which, if you are curious, can be found here.

Basically, you need to make sure that all the pieces are properly clean, so it is better if you wash them by hand. Use water no hotter than 104°F / 40°C. Otherwise, you could damage your LEGO. We recommend adding soap and cleaning them either with a sponge or with a toothbrush.

How to ensure your LEGO pieces do not directly damage fish

The third hazard is LEGO pieces that can directly damage fish. This is not about LEGO itself, but more a piece of general advice on decoration that is also worth keeping in mind for other elements.

Small pieces can be confused with food

Fish are not the sharpest knives in the drawer, so they can easily confuse small pieces with food. Therefore, do not place stand-alone small pieces. Fish could easily confuse them with food and try to eat them, which could easily make them choke to death. Not a view you want to have!

Pointy pieces can hurt your fish

In addition to small pieces, also be aware of pointy pieces. I have seen minifigs with a trident inside an aquarium and I always think how easy it would be for a fish to swim too close to that and hurt itself. Fish skin is not extremely thick or resistant, so if a fish swam too close to a pointy piece and touched it, it could hurt it and even kill it.

Therefore, when placing your LEGO inside the fish tank, make sure that there are no pointy pieces that can damage your fish. The edges of the bricks can also be dangerous, but the risk is much much lower with that than it is with, for example, a sword.

Which LEGO pieces can’t go into an aquarium?

The last hazard to keep in mind is a hazard more to the LEGO itself than it is to the fish, but it could potentially also damage your fish so keep in mind. In this article from LEGO, they highlight which elements should never be put into water. These are electronic parts and assemblies and mechanical components.

As far as electronic parts are concerned, motors, sensors, lights, and battery boxes from LEGO are not designed to be waterproof. So, if you put any of these inside water, you will most likely damage them and it could even become a safety hazard. This may sound logical to some of you, but it is good to highlight it again. No electronic parts should be put inside your fish aquarium. So, if you were thinking that it would be nice to have some flashing lights or something to create a current in your tank, forget about it. These parts of LEGO should not, under any circumstances, be used as a decoration.

Assemblies and mechanical components are a slightly different story. In this case, there is no safety risk to put them inside your fish tank. However, with parts that are made of multiple smaller pieces that can’t come apart, putting them in water is not a good idea. Why? Because the water will eventually get inside the part and get trapped there. It will never dry, which will create mold, which will eventually damage your LEGO.

Examples of these mechanical components are any type of Technic gearbox or DUPLO® vehicles.

How to make LEGO even safer for an aquarium

If you want to be completely sure about putting LEGO inside the aquarium, there are two extra steps you can take in order to make the use of LEGO even safer. These steps are painting the LEGO and only using new LEGO bricks. Let’s have a look at each of them and see why they will help in making LEGO even safer for a fish tank.


Paint used as a barrier can be an excellent way to make LEGO even safer. Obviously, only the right type of paint (or varnish) will do the job. Using the wrong one could actually make your LEGO unsafe, so it is important to pick the right one.

Our advice would be to use the Krylon Fusion Paint. It is the standard recommendation for ornaments of aquariums and it fixes well in plastic, so using it on your LEGO will certainly do the trick. You can pick whichever color you want or you can select the clear one in order to keep the current color.

Use only new bricks

The second step you can do to make LEGO even safer for your fish tank is to only use new bricks. Why is that? Well, the standards have gone up with time and LEGO has made their pieces safer and safer with the years. Therefore, using new bricks will ensure that they are of the highest quality and they are even less likely to damage your fish.

The downside of this is that, as we will see below, you might need to throw some of your pieces after having them in the fish aquarium for a long period of time. So it is a pity to use new bricks for these and then to have to throw them. Let’s see what we can do to minimize that risk.

How to maintain LEGO pieces that are in the aquarium

So, as we explained in the section on how to fix your LEGO, depending on the fixing system you use, you could potentially damage your LEGO. So, keep that in mind if you want to make sure you can reuse your LEGO pieces after they have been in the aquarium.

Periodical cleaning is required

In addition to that, the most important tip to maintain your LEGO pieces is periodical cleaning. The more often you clean them, the less dirt they accumulate, the easier it is to clean them every time, and the more likely you are to be able to reuse them after.

The exact time in between cleaning will obviously depend on how much time you have, how much patience you have, and how much you want to reuse the LEGO.

Our recommendation is to clean them once a week. I am aware that this can be very time consuming, but this way you will make sure that not too much algae grows on your LEGO and that it is easy and fast to clean.

I have also seen other people recommending every two weeks or even every month. Just keep this in mind, though.  The longer in between cleanings, the more difficult it will be to clean, and the most likely that reuse will not be possible.

Just clean them using the same system you use to sanitize them before placing them inside the aquarium and you will be fine.

Accept that, after using your LEGO in the aquarium, you might need to throw some pieces

But, as I mentioned before, please be aware that you might have to throw some pieces after having them in the aquarium for a long time. This is especially true if you do not do any periodic maintenance.

So, if you are going to put LEGO in your aquarium, just make sure it is LEGO that you wouldn’t mind throwing if you really had to. This will avoid surprises and regret further down the line.

Even LEGO uses LEGO pieces in their aquariums

If you are still not convinced about LEGO being safe for your fish, this last piece might just do the trick. Did you know that LEGO themselves use LEGO pieces in their aquariums? Would they be doing that if it was not safe? Obviously not. You can see below a few pictures from different LEGO aquariums.

Legoland Deutschland

Final words

And with this, we have arrived at the end of this post. If you enjoyed this post, you might also like “Are LEGO Safe For Pets?”, where we go through the safety issues of LEGO with pets in general.

Learn More

If you are interested in any big Lego ideas, then check out this Lego Ideas book on Amazon.