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I know this might be a very strange queation to ask but I was wondering if coal & more so sea coal. My tank is a SA set up & has black/gray gravel which I really like as it makes the fish & plants stand out but i'd like some black rocks/stones & wet coal is just the right colour. Now I can get quiet a lot of sea coal as it often gets washed up on the beach near me.
I was thinking of letting it soak for a month or 2 with daily water changes to remove any released salt etc. The Sea coal thats gets washed up on the beaches here in the NE of the UK is either from coal seams out in the North sea or it's the waste coal that was dumped into the sea many years ago when we had lots of mines. The coal you find on the beach is hard like rock & quite shiney unlike the dull soft coal that come straight out of a mine. The lumps you can find on the beach I would have thought would have been in the sea for at least 20-30 years if not longer. Due to the length of time in the sea I would think/hope that any sort of oil etc would have been released into the sea.
Like I said it's a strange queastion

Coal will not only leach bitumen as it breaks down, but will often contain high sulphur and metals. In a closed system like an aquarium, I'd avoid it. If you want black rocks, black basalt would work much better and that should be readily available most places.

Thanks for that I wasn't sure so thought it would be best to ask before putting any in the tank but will have a look to see if I can get some black basalt