Depending upon the thickness of your hunk of wood would depend upon how much leaches out of it. The bigger the hunk of wood, the more that will leach out.
For the mites, soak the wood in a 9parts water 1 part bleach solution overnight. Bleach kills =everything=. After the bleach soaking, soak the wood in dechlorinator water, adding liberal amounts of whatever dechlorinator you use, and let soak for a good two to three days with daily water changes.
Tea colored water in a tank is not bad. Continually changing out the carbon in your filter will help remove it, and frequent water changes will keep the color dilute.
Wood usually recommended to stay away from are softwoods (aka pines), ceadar, willow, and witchhazel. The last three have toxins that if they leach into the water could kill your fish.
~~Colesea