Although wood makes for beautiful, natural decoration in an aquarium, and more importantly encourages the growth of beneficial bacteria, it also has the potential to bring harmful fungus into the tank. The white mould saprolegnia in particular may appear on aquarium-ready bogwood. Tank owners should prevent this mould from spreading, and the keys to removing any kind of mould from an aquarium are care and patience.
Though fungal growth should never be encouraged or ignored, it's important not to take drastic action immediately: Adding toxins or boiling the wood may do more harm than good. Instead, aquarium owners should first try placing stronger light over the tank. Light encourages the growth of algae, which will attract protists, microorganisms that will also consume the harmful fungal spores.
Peat filtration is another way of attacking the fungus. Adding peat to the water filter increases the amount of tannins in the water, and tannins will reduce the amount of fungus and bacteria. Because bacteria can also be beneficial, this isn't a complete solution, but it may slow the growth of harmful organisms.
When fungus is very aggressive, the best action may be to lightly brush the mould off the wood with a toothbrush. Performing this action underwater, with a siphon drawing up all the spores released by the brushing, is the most efficient way to do it. It's important to be careful not to brush the unaffected areas of the wood, as this will remove beneficial bacteria.
It is tempting to apply polyurethane varnish to wood in order to prevent fungal growth underwater, but the varnish will not last forever, and in the meantime it will deter the growth of beneficial organisms that clean the tank naturally.
If wood fungus in an aquarium becomes a serious problem and ends up consuming too much time, another option is simply to purchase fake wood composed of cast resin or ceramic. These provide similar pleasure without the risks that biological material bring to the tank.