What’s the easiest type of stone to create good hardscapes

[RockMongler]

Boiling some rocks can make them explode, but most rocks you can boil just fine.  I would mostly worry about rocks that are very porous but with poor permeability (AKA, the rock has lots of open space inside, but those spaces are not well connected).  So things that appear very smooth, like stream cobbles, are perfectly safe to boil.  Sandstone, some lava rocks, rocks with a sandy texture and obviously layered rocks can have a less fun time.

As a geologist, I am comfortable picking rocks up out in nature to use in my tank, but it's often more complex than you might think.  @Chick-In-Of-TheSea the video you posted will look at rocks with carbonate reactions, which can be good or bad in water depending on your species and yoru tap water.  However, it will miss things like sulfide minerals.  Sulfide minerals (things like pyrite or galena) can weather to put metals and sulfuric acid in your water column.  And these minerals are not always obviously visible in a hand sample.  Nothing widely sold for aquariums will have these sulfide minerals, but if you are out collecting samples yourself, you might run into unexpected stuff.  In my 20L, I have some stream cobbles I collected at the headwaters of the San Juan river in colorado.  They are a mixture of rhyolite and tuff (rhyolite is compositionally like granite, but very fine grained and tuff is compressed volcanic ash).  I'm comfortable enough with identifying the constituents, but for most people I would be cautious picking up rocks to use in your tank because unless you are good at identifying minerals, you might miss something and have a slow killer in your tank. 

For example, in the attached photo, I have a large piece of what most people would call Granite.  it has a fairly smooth texture, not a lot of sharp edges, and looks quite pleasant.  However, in a portion of it, there is a fairly reasonable amount of pyrite.  Pyrite is made up of Iron and Sulfur bonded together.  In the environment of a fish tank, that could start weathering into lots of iron in your water, alongside sulfur compounds like sulfuric acid.  However, there is nothing that would necessarily react immediately to a pH change like some of the carbonate minerals shown in the video. Those chemicals would also not react to Gypsum, which is also known as selenite.  Gypsum is hydrous Calcium Sulfate, and if you put it in your water, it will dissolve into increased calcium and sulfate in your water, which most of us would not want in our tanks.  Some bacteria will eat that excess sulfate and produce hydrogen sulfide and other sulfur compounds.

Also, sand is a grain size term, not a compositional term.  Quartz is a mineral name.  Much sand like you find on a beach is quartz, but you can have sand made of any material.  Most beaches will be primarily quartz, but often have small amounts of other minerals present as sand as well.  White Sands National Park has entire dune fields full of sand that contains almost zero quartz, and is almost entirely Gypsum.  You can also have sand entirely made out of crushed up coral.

Most commercially available materials sold specifically for aquariums will be perfectly fine to use with little risk other than hardening water, or having sharp edges that might not be ideal for all species.  If something has been tumbled about in a stream, it is probably fine, but could possibly contain minerals you don't necessarily want to have in a small, relatively closed system like an aquarium. 

If anyone wants to take and send me good, clear, in focus shots of rocks they want to use in their aquarium, feel free to message me and I can take a quick look and help inform you of what it might be and what risks it may pose.

[Chick-In-Of-TheSea]

On 9/14/2022 at 2:05 PM, RockMongler said:

Boiling some rocks can make them explode, but most rocks you can boil just fine.  I would mostly worry about rocks that are very porous but with poor permeability (AKA, the rock has lots of open space inside, but those spaces are not well connected).  So things that appear very smooth, like stream cobbles, are perfectly safe to boil

I got away with boiling river rock.  🙂

On 9/14/2022 at 2:05 PM, RockMongler said:

However, in a portion of it, there is a fairly reasonable amount of pyrite. 

Fool's gold!

On 9/14/2022 at 2:05 PM, RockMongler said:

Also, sand is a grain size term, not a compositional term.  Quartz is a mineral name.  Much sand like you find on a beach is quartz, but you can have sand made of any material. 

Makes sense!

On 9/14/2022 at 2:05 PM, RockMongler said:

If anyone wants to take and send me good, clear, in focus shots of rocks they want to use in their aquarium, feel free to message me and I can take a quick look and help inform you of what it might be and what risks it may pose.

Thank you so much @RockMongler.  What a great post.

[Janoš Bećar Pecaroš]

I found that the best aquarium rock is a free rock. Buying substrate and decorations for aquariums can get very expensive very quickly. To rest the rocks if they’re safe for fish I pour some muriatic acid. If it fizzles I don’t use it.

[RockMongler]

On 9/14/2022 at 2:21 PM, Ištvan Bećar Pecaroš said:

I found that the best aquarium rock is a free rock. Buying substrate and decorations for aquariums can get very expensive very quickly. To rest the rocks if they’re safe for fish I pour some muriatic acid. If it fizzles I don’t use it.

Muriatic acid is a commercial term for Hydrochloric Acid.  It is the default method for testing for carbonate content in rocks as a geologist!  When I go into the field, I always try to carry a little bottle to let me know if there is any carbonate minerals in a sample.

However, muriatic acid will only tell you if it has carbonate materials, and won't fizz up with some other (admittedly far more rare) earth materials that won't be great for your fish.  

If anyone wants to add hardness to their water, rocks that react with an acid like that will very likely have a similar effect as putting in crushed coral or one of the other options people often use for adding hardness.  Just NEVER use green or blue rocks that react to an acid like that; they are probably copper carbonates like azurite or malachite, and could have negative effects on your livestock.

[Janoš Bećar Pecaroš]

On 9/14/2022 at 3:03 PM, RockMongler said:

Muriatic acid is a commercial term for Hydrochloric Acid.  It is the default method for testing for carbonate content in rocks as a geologist!  When I go into the field, I always try to carry a little bottle to let me know if there is any carbonate minerals in a sample.

However, muriatic acid will only tell you if it has carbonate materials, and won't fizz up with some other (admittedly far more rare) earth materials that won't be great for your fish.  

If anyone wants to add hardness to their water, rocks that react with an acid like that will very likely have a similar effect as putting in crushed coral or one of the other options people often use for adding hardness.  Just NEVER use green or blue rocks that react to an acid like that; they are probably copper carbonates like azurite or malachite, and could have negative effects on your livestock.

Yep, it’s HCl diluted with water. I just pick everything I like and the ones that fizz go in the garden.

Its true that muriatic acid only reacts with carbonates, but other nasty stuff is hard to find and are often colored brightly or have metallic sheen. I’m not a geologist, but I used to collect rocks and took some formal classes in school so I’m pretty confident in my ability to avoid anything dangerous most of the time. Well, I haven’t killed anything yet due to dropping some rocks in my tanks.

My friend from Serbia claimed that he used copper sulfate salt (very pretty blue crystals) to treat parasitic and fungal illnesses in his aquarium. However, I’ve never tried it and I don’t think I ever will.