[danmd]

Are these rocks sandstone? Can I use them for my tank? Thanks for your help

Moreover, they have some black stuff on them

Please let me know if they are safe


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[PlantedRich]

The first is almost definitely a sandstone as you can almost see the sand. The second "might" be a really eroded piece of limestone, although I would lean to sandstone. Sand stone is normally quite safe as it really is just old hard pressed and heated sand.
Odds are you are good. But if you did have really acidic and soft water, adding limestone would be likely to change your PH and GH/KH. On the other hand , if you have high PH, hard water from your water running through limestone most of it's life, there would be no change expected. Like adding black ink to black paint?
So there is no one firm answer to what rocks will do to tank water. Each tank is different.

 

[danmd]

PlantedRich,
Thanks for your information. Since the sandstone would be degraded in water,I just threw away those sandstones and got some of others. Do you think these stone will be Ok?

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[PlantedRich]

Other than the potential for rocks to change the PH/GH/Kh of your water, most are safe to useif they are not polluted by something they have picked up along the way like oil,pesticide, etc. There are a tiny number which can have too high metal content but they are pretty rare in most parts of the country.
Sand stone may not erode as quickly as they sounds. They are often hard enough to set outside in the weather for many years.

 

[metageologist]

PlantedRich,
Thanks for your information. Since the sandstone would be degraded in water,I just threw away those sandstones and got some of others. Do you think these stone will be Ok?

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most of those rocks should be fine including that quartzite. Tho only rock I don't like the looks of the lone rock in the lower right it appears to be a shirt and without a close look I can't tell for sure but the rest look fine

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[Diana]

Very easy to run your own test.

Put each rock in a separate container with some of the water you will be using in the tank.
Test GH, KH, pH, TDS when you start, then every few days for a week. The most highly active rocks will alter the water within a day or 2.
The less active rocks might take a week or longer to do anything, and it might be OK, small changes that you will correct with weekly water changes.
Some will produce no change over the life of the aquarium.

As pointed out by Rich, in his first post, if you already have hard water, then the rocks are not going to make enough of a difference to worry about.