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What Rock can I put in my Aquarium? – Aquarium Rock quick guide

What Rock can I put in my Aquarium? - Aquarium Rock

Aquarium rock – It is common to see debates over what rocks can be placed safely into an aquarium. There are tales of dissolving rocks, poisonous rocks and even explosive rocks, so which are actually safe? Our answers may surprise you.

What Rock can I put in my Aquarium?

As an aquarist, I have always been aware that certain Rocks should not be put in the aquarium. I’ve seen loads of rocks in local rivers that would have looked great in my tank, but I have always avoided them as I am no rock expert and would be unable to identify exactly what they were.

Many aquarists have thought this for years and you will regularly see lists of rock that are deadly to your aquarium fish. This is why many aquarists spend a fortune on rocks from aquarium shops, as that rock is deemed ‘safe’.

But this week I got into a conversation on this with someone much smarter than I am and was slightly more educated on the topic. So now I’m writing this to pass a bit of knowledge to you:

“Almost all Rocks are safe for the aquarium”.


This may come as a surprise to many, but if you think about it, it makes sense.

All those rocks I saw in my local river were in constantly flowing water, nothing had happened to them, nothing significant anyway, and the river was still teaming with fish.

It may be argued that the river carried any toxins away, but rivers are full of rocks, of all different kinds, the whole way down them. Is all that toxicity somehow missing every fish on its way past?

No, there just isn’t any toxicity.

So, if you’re asking, “What rocks can I put in my aquarium?”, the answer, is any!

The only exception to this is when keeping sensitive fish like wild-caught Discus. Discus, particularly wild-caught specimens can be very sensitive to any changes in water parameters etc, so a biotope set-up works best in this case.


What Rock can I put in my Aquarium? - Aquarium Rock


The effects of Rocks in the Aquarium

-What Rock can I put in my Aquarium?

With the vast majority of rocks when you add them to the aquarium, absolutely nothing happens other than the water level rising a bit.

Most rocks are fairly inert and will not have an effect on your water parameters, some though will have a small effect.

Calcium Carbonate in aquarium rock

Calcium Carbonate is found in many types of rock, rock such as limestone or aragonite contains fairly high levels.

This is what the ‘vinegar test’ is supposed to indicate, but I’ve tried it and never found that the vinegar fizzes, this is because a much stronger acid is required to cause a visible reaction.

Calcium Carbonate can raise pH and this is one thing to monitor when adding a rock to your aquarium. If you have an aquarium with low pH and low hardness, adding a rock such as limestone can cause your pH, KH and GH to rise, but your pH will not go higher than 7.9.

The reason this may need to be monitored is that sometimes, large changes in pH can cause pH shock, but this is rare.

7.9 is absolutely fine for most fish, but blackwater species such as Neon Tetra will not breed in these conditions.

The lower your pH is, the larger a change will be seen. This is because acidic water will be more effective at dissolving the Calcium Carbonate contained within the rock. The pH will still not exceed 7.9 though.

If your pH is already high, 7.6 or higher, it is unlikely that you will notice much if any difference.

Calcium Carbonate will also increase your KH, this means that your water will be far more capable of holding a stable pH. If you have a low KH value then adding rock containing Calcium Carbonate may be a great idea.

Many people use it as a substrate (Aragonite coral sand) for this exact reason.


What Rock can I put in my Aquarium? - Aquarium Rock


What Rock is poisonous to my aquarium?

-What Rock can I put in my Aquarium?

You may be wondering, why there are all these lists of rocks that will kill your fish if it simply isn’t true?

Some forms of rock do contain poisonous minerals. There is a list of them, but there isn’t much point in listing them here. They are incredibly rare and most are only found deep in mines and are prized by collectors, so to come across one in your local river is pretty unlikely, to say the least.

If you do want to see a list, check the references at the bottom of this article (Aquarium science).

Ultimately the chance of finding a poisonous rock in your local area is almost zero. So the only real concern to have is if you are breeding a blackwater species and need to maintain a low pH.


Stones that Dissolve?

-What Rock can I put in my Aquarium?

Stones dissolve in the aquarium? This one has always puzzled me. If Rocks dissolved in water, then were they all just made yesterday and will disappear the next time it rains?

I mean, it rains constantly here in the UK so even rocks that arent in rivers, if dissolvable, should be long gone now. Yet there is sandstone everywhere.

Some aquarists say that soft rock, like sandstone, will simply dissolve in the aquarium. This is not true, however some rocks will break down over time, particularly in acidic conditions. But rocks will not simply turn to dust before your eyes, nor will they create a toxic environment for your fish.

Rock like Sandstone is perfectly safe in the aquarium and will not poison the water, cloud it or make it toxic. If you keep the same piece in your tank with high water flow passing it for 50 years, you may see some small changes, but that’s about all.


What Rock can I put in my Aquarium? - Aquarium Rock


Gemstones and Quartz in the Aquarium

-What Rock can I put in my Aquarium?

Many aquarists will say that Gemstones and Quartz are dangerous for the aquarium too. This again isn’t true as Gemstones and Quartz are inert, they don’t dissolve in water and most will only break down in very strong acid.

So, any concern with Gemstones and Quarts when adding them to the aquarium would be caused by whatever has happened to them since they have been removed from the ground.

If they have been bought from a shop as an ornament then it is possible that they have been treated, polished or a number of other things. So whilst Gemstones and Quartz are safe, you will need to be fairly sure what has happened to it afterwards before throwing it in the tank.


What Rock can I put in my Aquarium? - Aquarium Rock


Cleaning Rocks for the Aquarium

-What Rock can I put in my Aquarium?

When cleaning rocks to add them to the aquarium we all know that we need to boil them or wash them in bleach, right?


There is absolutely no need to boil or bleach rocks, in fact, bleaching them is likely to do more harm than good.

Bleach and other detergents are far more harmful to your fish than anything you may find on a rock naturally, so if you miss a bit when rinsing it off, it may kill your fish. Then the rock will take the blame and the cycle of rock myths continues.

If you want to add a rock to your aquarium, a rinse in fresh water and a good brush should be all that’s needed to remove any dirt and debris. There is no need to boil it but it WILL NOT explode if you do.



-What Rock can I put in my Aquarium?

If you have seen a cool looking rock in your local river recently and fancy adding it to your aquarium, go for it. Unless your local river has some refinery dumping waste into it there is very little that could go wrong due to the rock itself.

There seem to be a lot of Myths about the adverse effects of rocks in the aquarium that over time have become ‘well-known facts’.

The only factor worth noting is that some rocks can raise your pH. But none will raise it to dangerous levels unless you are keeping very sensitive fish such as wild-caught Discus.

Do be aware though that heavy rocks and glass do not mix well. So be careful not to damage your aquarium with them!


Happy Rock hunting!!


Frequently asked questions:

Can I put rocks from a River into my Aquarium?

Yes. The chances of finding a rock that will cause issues in your aquarium is pretty slim. Just give it a good wash first. Some rocks may increase pH, so if you are breeding soft water fish, check the rock type first.

What rocks are Aquarium safe?

Most rocks are perfectly safe for the aquarium. Rock/mineral types that leach toxins into water are very rare and are often found deep underground.

Will rocks dissolve in the aquarium?

Rocks do not dissolve in water, however, rock types like sandstone will degrade over time. They are still safe for the aquarium and will take many years to break down.



Aquarium Science 14.3 – Rocks

Carbonate Chemistry


About the author

James @Seriously Fishy

I am a fish keeping enthusiast with over 20 years experience. I currently keep American Cichlids (CA) which are my favourite fish to keep so far. I started Seriously Fishy as I noticed a large volume of people on various web pages looking for help with Aquarium basics. I created the first Seriously Fishy book to solve the issue in 28 pages, that led to the Seriously Fishy UK fish forum and blog.

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