Natural wood in an aquarium looks beautiful and can make an aquascape look more natural. There are also biological benefits to having real wood in an aquarium. Most aquarium owners who put wood in their aquarium prefer the wood stays at the bottom; this can be a problem since most wood floats in water.
Getting your aquarium wood to sink in water can be simple but not always. Many types of wood will sink after they are allowed to become saturated with water. Some common aquarium woods, like Manzanita, are popular in part for their ability to sink after saturating.
To saturate your wood you can soak it in water for a while. Keep some kind of weight on the wood to keep the entire piece submerged. The soaking will also help to leach tannins out of the wood which can discolour the water and can be harmful to some fish. The time needed to saturate the wood will vary depending on the size and the type of wood. It is a good idea to change the water every couple days when soaking if you are also wanting to remove tannins.
Another way to get aquarium wood to sink is to weight the wood in your aquarium. Enough extra weight can get the wood to sink and eventually it will probably saturate enough that you can remove the weight if you want. If you have rocks in your aquarium you can wedge the wood between them or place a rock on top of the wood. Beware that if the rocks slip the wood will then float to the surface. Both the wood and rocks moving could damage your aquarium and injure your fish.
A better way to weight aquarium wood is to attach a weight to your wood. This is often done by using silicone to glue the wood to a rock or piece of slate. When using silicone make sure it is non toxic or food grade silicone. You can also drill a hole in the wood and insert a piece of stainless steel, rock or other non toxic heavy material. You can wedge it in or use silicone to glue it in. For those who prefer not to have any metal in their aquarium, you can use a piece of fishing line to attach the wood to a heavy object like a rock.