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Discussion Starter · #1 ·
So here's the story, It seems like there are many conflicting opinions on what rocks are "safe for aquarium". in the research I've done it seems that rocks that dissolve in acid (bicarbonates) and tht can be etched with a stainless steel knife are bad.
However, what if a rock is just able to be etched by a knife?
For example, I just ordered some slate Small World Slate & Stone
and on the home page they have an image of a little shrimp on their stone, stone is covered in algae obviously been in use for some time. The video says that they are inert and PH neutral.
However, when I got my shipment these slate pieces are extremely weak and able to be basically etched by a knife no problem.
So what does this mean for a tank then? Are stones that are able to be etched by a knife actually safe for aquarium? Why then does the community say you shouldn't use rocks that are able to be etched?
Wondering what others think, obviously setting stones in water for a week is the most fool-proof test but just wanted to start a discussion on how people came to these testing conclusions and whether they hold up 100% or not.
 

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A stone can be soft enough to etch with a knife and still be safe. Etching is just an indicator of density. I can etch lava rock, and people use that all the time.

Solubility is the big thing. A mild acid reacting to it will tell you if something is likely to be soluble in water over the long term.
 

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Discussion Starter · #3 ·
Thanks for the reply! I will start commenting on posts I see on the internet about etching then, that makes good sense.

A stone can be soft enough to etch with a knife and still be safe. Etching is just an indicator of density. I can etch lava rock, and people use that all the time.

Solubility is the big thing. A mild acid reacting to it will tell you if something is likely to be soluble in water over the long term.
 

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Pour a couple of drops of vinegar on it. If it reacts (fizzes) then they are NOT safe. If you’re going to soak them, and test the water after 24 hrs, then a couple of days later - make sure to test your water for reference before adding them. There are different forms of slate and can have added minerals mixed in, an obvious sign would be iron or rust spots, which would not be suitable. If you are getting it from a commercial supplier, you don’t have the advantage of picking and choosing your pieces, but if they said its inert, then it should be - you probably just didn’t get their top shelf stuff...
 

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Thanks for the reply! I will start commenting on posts I see on the internet about etching then, that makes good sense.
OK lets go back to some old geology stuff..
Shale is the precurser to slate.
Slate has a hardness of 2.5 to 4
Shale about 3.

Streak plate teste of shale is white
Slate light to dark brown.

Reason one needs to know the difference:
Shale can contain a lot of oily substances.

The "streak test" is a method used to determine the color of a mineral in powdered form. ... The streak test is done by scraping a specimen of the mineral across a piece of unglazed porcelain known as a "streak plate." This can produce a small amount of powdered mineral on the surface of the plate.
Shale:


Slate:


Background
Shale and slate are sometimes used interchangeably, but the materials are not the same. Many of the items sold today for landscaping (flagstones, retaining walls) and construction (chalkboards, roofing tiles, pool tables, etc.) uses as "slate " is actually the much more inferior form—shale. Shale is sedimentary rock, lightly compacted into thin, crumbly layers. It may contain high concentrations of quartz, feldspars, pyrites amorphous silica, and clay minerals: illite, kaolinite, chlorite and smectite (montmorillionite). This smectite clay is a highly absorbent component that can swell to several times it's normal size, making the shale loose strength. Slate, a metamorphic rock, on the other hand is much stronger and does not absorb water (remember how fast those school blackboards would dry when you had to stay after class?) Generally, it has much greater strength. However, certain types of slate still may be less than desirable. There also is a high calcite/dolomite form of slate called Marl Slate, from Durham, UK, which can be softer and easily attacked by acids. Shales, a much cheaper material, may be sold as slates at higher prices with a substantially reduced strength and product life.
 

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At one time aquarium bottoms were made out of slate.. We have 4 pieces of slate in the bottom of my daughters Axolotl tank currently and I have used slate a lot in the past. I believe slate is very safe for aquarium use.
 

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I think when people are talking about “etching”, they mean chemical etching, as in the marks that acids leave on the rock, as opposed to physically scratching the surface with a knife or other tool.
 
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