Wood is naturally buoyant. That's why you see some driftwood with large slate weights on the end in pet stores. However, if it is submerged for long enough, it becomes filled with water (waterlogged) and will sink. This process can take a fair a mount of time, and if the wood is removed from the water for too long, it will dry out and need to be re-soaked.
Wood also contains tannins. Tannins will turn your water brown and acidic. Although not necessarily bad for certain types of fish and plants that are used to living in tropical rivers and bogs (where wood routinely falls into them), it can be unpleasant to look at and can smell (like a bog...naturally).
Finally, wood will eventually rot in water. Different types of wood rot at different rates, and in general, the harder the wood, the longer it will take to rot. The hardest woods can last for hundreds of years underwater (like oak), but softer woods can begin to rot in just a few. You should be fine as long as you don't choose a really soft wood, but just keep this in mind, and don't choose a piece that is already rotting on its own!
All three of these issues can be overcome by sourcing your driftwood from a lake or river directly. It is usually already water logged, the tannins have leeched out, and you know it won't rot too quickly. All you need to do is sterilize it by boiling it.
However, if you can't get your wood from a lake or river, here are some tips and methods to make it safe for your aquarium.
- Choose wood that is hard wood. Stay away from pine, cedar, and elm! Also, weathered, unique wood makes for a better aquascape than a hunk of wood.
- Wash any debris or bark off your wood.
- Boil your wood for 2-3 hours. This will accelerate the process of water logging the wood and leeching the tannins out. It will also sterilize the wood and prevent any unwanted guests from wreaking havoc on your aquarium.
- Soak your wood in a container of water for about a week before putting it in your aquarium to let the tannins leech out. This will also help to water log it so it won't float to the surface. You may need to put a few rocks or something heavy on top of the wood to keep it submerged. The water will turn a light brown color as the tannins leech out.
- Once your wood sinks naturally, rinse it off and it is ready to be put into your aquarium. If it is still not sinking after a week, continue soaking it until it does. The amount of time it takes varies on the density of the wood and how large of a piece it is.
Enjoy your free driftwood!