Earth stones

jaguar and convict cichlids are central american, not south american .. and such rocks would be fine. green terrors probably fine to if already in hard alkaline water ... and could be easily acclimated to those conditions if not.
 
The vinegar thing works if the rocks are really calcerous; I once did it on Cichlid Sand and it fizzed like soda pop. But for those rocks that are low in calcium content, you need a stronger acid.

Someone mentioned that one of the bottles in the AP nitrate test kit would do the trick, though I forget which one of the two. (Someone know?) And it worked for me.

One more thing - make sure that where you get your rocks is not a place where the potential for chemical fertilizer or pesticide runoff occurs. That stuff is nasty and I'm not sure i would trust it after even a bleach boiling.
 
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