Using Driftwood in Aquariums

What tannins do, how to prepare driftwood by boiling and soaking, why wood floats then sinks, safe wood types, and the harmless biofilm that appears.

Tannins

Tannins are astringent polyphenolic compounds found throughout the plant kingdom, abundant in tree heartwood and bark. As driftwood releases tannins it tints the water a tea-brown colour and can lower pH and add mild acidity. This is the same process that gives bog water its characteristic brown stain. Tannins are not harmful in themselves; for soft-water species from blackwater habitats they recreate natural conditions, while others may simply find the colour undesirable.

Why wood floats first

New driftwood usually floats because it has not yet absorbed water. Over time the wood becomes waterlogged as it saturates, and eventually sinks on its own. Soaking accelerates this, and a piece that still floats can be weighted down or attached to a rock or slate with aquarium-safe silicone until it sinks.

Boiling

Boiling helps sterilise the wood and speeds tannin release. Submerge the piece and bring it to a rolling boil for at least one to two hours, discarding and replacing the darkened water. Boiling also helps the wood begin to absorb water so it sinks more quickly.

Soaking

After boiling, or as an alternative for pieces too large to boil, soak the wood for several days to weeks, changing the water regularly. Extended soaking draws out the initial burst of tannins and organics so the display tank is not flooded with them at once. Different woods leach at different rates; some release tannins in a rapid early burst while others release them gradually.

Safe wood types

Choose dense, hard woods, which decay slowly and produce little debris. Softwoods such as pine, cedar and fir contain resins and oils that can be toxic to fish and should be avoided. Driftwood sold for aquarium use is selected to be safe; collected wood should be hardwood, free of rot, sap and chemicals.

Biofilm

In the first week or two a slimy, white, cobweb-like film often grows on new driftwood. This biofilm is a harmless bacterial and fungal bloom feeding on organics released by the wood. It usually fades on its own and is readily grazed by shrimp, snails and many fish.