How to Use Rocks From Nature for Your Fish Tank

One of the hardest things regarding furnish a fish tank for me is to choose and get the right rock. I find it hard to find the right shape and visualize how it will fit my fish tank.

You could put rocks from nature in a fish tank. Be sure it’s hard enough and doesn’t contain minerals that are possibly harmful to your fish. Always rinse and disinfect your rocks before putting it in the fish tank.

There are some precautions you need to take before picking a rock. Be sure to follow these steps to not risk harming your fish in your fish tank when picking rocks from outside.

Surrounding area

The first to think about is the area you are picking your rocks from.

Be sure not to pick it close to any industrial area. There might be heavy metals or other nasty things coming from the industry which will get into the groundwater and maybe into and on the rocks in the surrounding area.

It’s a good idea, but not necessary, to pick rocks near a water source. That’s because you are able to rinse the rocks to look for signs for rock not suitable in a fish tank. It will be messy and heavy if you bring your own water to rinse the rocks.

If you don’t like the look of rocks being formed by the water running by them you could go into the wood and find other shapes of the rocks then you can find nearby water. You might also live much closer to the wood than to a water source which makes it easier for you to bring the rocks home for you.

It’s something to consider, especially if you ick heavy rocks or large quantities, how you bring them home. You probably won’t carry the rocks a large distance by hand.

Also, take a look at nearby places to see if there are different kinds of rocks in those places than what you have just outside your doorstep. It sometimes is a good idea to mix a different kind of rocks but still has the harmony of a well-planned fish tank. It’s nothing wrong with having just the same kind of rock though.

If you only have rocks from outside it’s easy to just have the same kind of rocks. By going to another part of the country you might find other rocks that are suitable for a fish tank. Just think of how you will transport the rocks.

Check if it’s legal

It might actually be illegal to pick rocks. Laws are very local so it’s best to look it up from where you live. For some, it might vary within the country so be sure to check the very local laws of where you are picking the rock.

You don’t want a fine, then it will be a very expensive rock you are picking. Then it will be cheaper for you to get the rock from the pet store.

Also, make sure you are not on private property picking rocks. If you are, be sure to ask the owner before that you are able to be there and if you are able to pick some rock for your fish tank.

Most don’t mind but there might be some that will say no and if they do, you need to be respectful for that decision and accept it. There is a lot of rocks in the world so you don’t have to be sad if you are not able to pick those rocks.

One more thing to be sure of is to not pick any rocks that have historical value. For example, if you are finding any rare rock or fossil you should not bring it home. Bring it to someone knowing what to do and take care of it.

It’s probably not you are going to find these since it’s rare but you never know! If you find that kind of rocks or other things, I have to bring it to a museum from where I am, so make sure to check from where you are.

Which rock is the best?

There are some things you are looking for when searching for the perfect rock in your fish tank.

When you look on the rocks on the ground, be sure to not look for a rock that’s glittering. While it might be beautiful in a fish tank you have to be sure to not pick them since it’s a sign it might contain heavy metals that will destroy any fish life in your fish tank.

Also when walking around looking at the ground for the rock you want to take with you home you need to be sure to have the right sized rocks.

To be sure of it is simply to measure the dimensions of your fish tank and write it on the ground and place the rocks inside the shape of your fish tank you have painted on the ground.

You could also bring rimming or something else resembling your fish tank. That way it will be more exact since you have the exact size of your fish tank. Either way is a viable way to be sure you don’t bring home too big or too small rocks for the vision you have.

Another thing that goes well with a lot of things is that if you are unsure, don’t pick that rock. It’s better to be safe than sorry and you don’t want to risk your fish lives just because you are uncertain.

Be sure to be certain that the rock you bring home is safe for fish tanks. take a look at how the rocks in the pet store are and have that as a guideline of what is safe and not when doing the ocular inspection of the rock.

Test the hardness of the rock

When you have spotted a good rock you need to do further testing to see if it really is safe.

One of these tests is to test if the rock is hard or soft. By doing so you make sure that the rock is not falling apart and risk hurting your fish and releasing stuff inside the rock into the water.

A knife is a good tool to use. There is a hardness scale of minerals that determines how hard the mineral is. A harder mineral is able to scratch or damage softer materials on this scale while a softer material is not able to scratch any mineral higher on the scale of hardness.

A knife is made of stainless steel and has a hardness of 7 out of 10 on the hardness scale. That means if the knife scratches the rock, it’s softer then 7 and if it’s not, it’s higher up on the scale then the knife.

Use the tip of the knife and scratch the surface of the rock. If it scratches it’s a soft rock, if it doesn’t it’s a hard rock. A softer rock runs the risk of releasing this stuff you don’t want in your fish tank.

If the rock is hard it’s generally safe to put it in the fish tank but if it scratches you need to test if it has any metals or carbon etcetera by doing the next step.

Test if there are minerals in the rock

The next step if your rock is a soft rock is to test if the rock contains metals or organic compounds that are harmful to your fish.

By doing so, you eliminate the last risk of the rock you picked outside to be harmful to your fish tank.

To test of minerals you just put on a little muriatic acid on the rock and see if it fizzes. If it does, you have a rock that contains minerals that might be soluble in water and potentially harmful to your fish tank fish.

Why you want to use muriatic acid (or hydrochloric acid that it also might be called) and not any stronger acid is that while it’s an acid and will dissolve the minerals you want to check for, it also disassociates in water. That means that the acid will dilute and not be harmful in those small quantities. That’s also why you should be near a water source, to wash the acid away.

You could buy it relatively cheaply from some stores but you might also have it in your cleaning cabinet. Some cleaning products are containing muriatic acid and those are viable options too. It’s not much needed for testing rocks.

Another acid that is widely recommended is vinegar. It’s probably because it’s acidic, easy to get a hold of and most people have it in their kitchen.

It will work to check rocks with vinegar but you will not get the clear result as you will get with muriatic acid since it’s a much weaker solution. Most vinegar is diluted and only containing some few percent of acid. That will make the reaction much harder to see because it won’t fizz as much as the muriatic acid will.

If you are going on the vinegar route, be sure to bring a magnifying glass to be able to see the bubbles when poured on the rock.

Since the reaction with vinegar is so weak, I would not suggest you use it for checking rocks. Use it for your cooking instead. If you use it for checking rocks, you will only wonder if there was a reaction or not and you might bring home a rock that might be harmful to your fish tank.

Putting the rock into the fish tank

Now you have found your rocks. You have brought them home with you and are ready to put them into your fish tank and starting to make it visually appealing once and for all.

Not so quickly!

You have to do some more things to your rocks before putting it into the fish tank. Since they were outside they might contain bacteria and other nasty particles making your fish sick if you get these into the tank.

To remove these microorganisms and particles you have two ways to choose. One is to use boiling water and the other is to use chlorine.

The boiling water method is the easiest and available for everyone having water and a kitchen. You simply bring water to a boil and pour it over your newly found rocks. By bringing hot water onto the rock you will kill off any harmful microorganisms that might live on the rock.

Do not boil the rock!

By boiling the rock you might make water trapped inside the rock to boil and expand making the rock exploding. That’s something very dangerous and messy for you so promise me not to boil the rock!

The other way to take is to soak the rocks in a bleach and water solution. Bleach is something you also might have in your cleaning cabinet for the purpose to kill microorganisms.

If you go with this route, be sure to leave the rock outside for a little while. I would say for a day, just to be sure. That’s because to give the bleach time to evaporate. It evaporates relatively quickly but to be sure to not bring any bleach into the fish tank, leave it out in the air for a day.

The last thing to think about before putting the rock into the water is that the rock will replace the same amount of water as its mass. So, be sure to take some water out from the fish tank so you don’t get an overflow and you having a mess to clean up instead.

Also, check the water parameters the first days after putting the rock into the fish tank to be sure nothing is happening. The rock, depending on the rock should do nothing to your water parameters but maybe raise the pH.


I have been a fish keeper a big part of my life. During the years I have gathered and searched a lot of information about fish keeping and here, on the website, is where I share that information to help you with your fish keeping. I want to provide the information that I didn't get when I had a question about my fish tank.

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