I'm curious about a good way to make this safe for my fish/shrimp tanks? My BF works for the Dept. of Fish and Wildlife and often finds chunks of petrified wood on the rivers he works on. He's given me a couple of nice chunks. So far I've only used a toothbrush and tap water on them. I'm sure to get more and will eventually start using some in my tanks. How can I be sure all the river gunk is gone?