Basics : Driftwood
Artificial driftwood (usually a type of plastic) is inert (does not change water parameters), and you only need to periodically check that the finish is not eroding.
If you have freshwater tropical fish, then collected wood from marine (ocean beach), or terrestrial environments (forests) is much safer than those collected from freshwater lakes and rivers, because any potential diseases (ie: bacteria) and parasites (ie: flukes) are less likely to have survived outside their normal environment.
Whether you are buying or collecting, here are a few things to look for in your selection, and in getting it to sink.
Natural driftwood is normally an attractive and beneficial addition to an aquarium
(and essential for many algae eaters which need the fibre to keep their gut clean).
Depending on many parameters, it will to some extent, react with the aquarium's
water, typically leeching a bit of tannic acid, and removing
some carbonates. Tannins may colour your water in a brown tint, and cause your
pH to drop, which depending on your circumstances may or may not be desirable.
Fungus, moulds and various algaes may also take root in your wood, and can be
removed mechanically or by boiling.
Natural wood may contain sap resin which in the close confines of an aquarium, may be toxic. Wood should be as dehydrated as possible to minimize this danger.
Details...
BUYING OR COLLECTING
The cost of driftwood in a store seems quite high when you think that it's just a piece of wood on a slate base. If you do use store bought wood, try to buy when they receive their delivery. The prices are by size & weight, but the best pieces (about 20% of the total) will typically be sold in the week following the delivery. The store driftwood I typically see here is mopani (heavier than water) and white or red western cedar (on slate).
For an idea of it's value, some stores import their driftwood from the far east. While retail costs are high, your time & trouble (locating, transporting, cleaning, drilling slate, boiling and rinsing), can add up as well. However, nothing compares with the satisfaction of doing it yourself, but you need to research, and have a good choice of materials. Otherwise, that $60 piece at the pet shop starts looking better at $5 a month for a year.
SELECTION
For selection, measure where the piece(s) will be going (the inside measurements
of your tank are not your outside measurements, especially the width & height).
Leave a few inches around the wood or push it directly up to the glass (small
gaps are bad). Any glass-wood gap should be about 50% wider than your thickest
fish. It is recommended to place the driftwood at the 1/3 or 2/3 points of your
tank's length (you don't want to draw your eye to the tank centre). Avoid pieces
with thin branches (they rot and fall off). Avoid over-thick chunks (may be hard
to sink, displaces too much water, may leech for a long time). Avoid pieces with
sharp points (hazardous). Canopy pieces (like archways) work with small fish which
can go under them (and then attach Anubius, Riccia, Java moss or ferns to the
wood. Blunt low pieces work better with larger fish. Sometimes I unscrew the slate
and re-attach them, so a 45 degree piece is now horizontal (makes good tiers to
hold gravel back). Try to put the slate on the glass bottom (or you may not have
enough gravel on top to weigh it down, or it might be insufficient to grow plants
above the slate) but watch the screw doesn't protrude too much (a small thin sheet
of styrofoam fixes that if you're worried), and any hardware (slate screws) should
be stainless steel.
SINK
A good dry piece of wood will be very buoyant. If the slate is not heavy enough, it might take a few days to sink. If you are not using a weight, it might take a very long time to sink (if it even does). A suitable container (certified for potable water) is used to soak your wood before putting it in the aquarium. After a few days, you can assess how resistant it will be to sinking, and how much tannins will be released (making the water brown). Tying the wood to an inert stone, such that it sinks to the bottom, will accelerate waterlogging it.
Boiling the wood will reduce its buoyancy (rupturing cells which then fill with water), but finding a container large enough to boil the wood is often a challenge (you can also do half at a time, with it partially out of a pot).
TANNIN ACID
If you have a low-pH soft water tank with tetras and Discus, then the effects of the wood are generally desirable (brown tinted acidic water), but as the wood looses its tannins, the effect will diminish with time.
If you have a high-pH hard water tank with African cichlids, then too much tannins will not be desirable. In an alkaline tank with high gH and kH, the effect might be negligible, so driftwood does not need to be avoided, but you should be aware of the effect if you are using a lot of it.
The rate at which the wood makes the water brown decreases with its age and can be significantly reduced by boiling (may take more than one boil). Very well dried wood will have far less of a tinting effect on the water. The tint itself is harmless. Stores sell Black Water Extract which is a concentrated version of this tint. The acidifying effect should be monitored in low kH tanks (those with a weak carbonate buffer of less than 3dkH).
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