05-10-2010, 08:54 PM
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Okay. I know people use natural slate in their tanks. Slate derives its colors from the sediment is forms near. The colors of slate are usually specific to that area. The slate you are showing resembles the kind mined in areas of the east coast. I do know slate was used in electrical work a long time ago (doesn't conduct electricity ((suggests no metals))). I have a slate topped pool table, but that has no bearing. Wikipedia says slate is chemically inert.
The problem with using these stones is you do not know what it has been treated with up until now. A lot of companies will treat or even dye their materials (slate is very easy to dye) to resemble something that it is not. (East coast slate sells for more that mid west slate. buy cheap, add some color, sell for more) They also may pre-treat it to make the surface more porous (for paving) or water resistant prior to putting out for sale. Slate is very susceptible to the elements, especially in areas of climate change. Pre-treatment protects the "stock" from damage. If this is done it may not be readily advertised or even obvious. A way to test would be to weight the stone while it is bone dry (baked) then submerging it for a few days and re-weighing it. If it is heavier, it means it is absorbing water. It is not a foolproof idea, but it is a start.
You also cannot be sure what chemicals or overspay it was exposed to in transit or anyplace else. For this, I'd recommend a very heavy series or soakings, bakings, and maybe a few trips through the dish washer on high heat. THEN test is in a controlled atmosphere on one or two fish.
I def. would not settle for a few day soak and toss it in a tank willy nilly... unless you're okay losing a tank and having to start from scratch.