Bombalurina suggested I make a post about choosing safe rocks to use in an aquarium, as I mentioned in another thread that some minerals are really toxic (I collect minerals as a hobby). While I’m at it, I thought I’d include other decorations as well, such as plastic and ceramic items.
Decorative items sold in pet and aquarium stores that are marked as being specifically for aquarium use are usually very safe. But not all objects you may find around the house or in nature are okay in the fish tank, as some will leach out toxins that will, sooner or later, become toxic to your fish.
So how can you make sure to pick safe items? One way is to use nothing that isn’t from a store and marked specifically for aquarium use. But you can also do a little homework about the likelihood of toxicity in other items, and make good choices based on that.
The following list isn’t 100% comprehensive, but might be useful in helping you choose safe decorations.
Basic common sense: if in doubt, don’t put it in your tank.
It’s better to be safe than sorry!
Plastic is one of the most-used materials in fish keeping. Many plastics are completely safe- the acrylic used in fish tanks, for example, and the materials used in plastic plants and other aquarium decorations. Some plastics, however, will degrade in water or release toxins when heated even slightly.
- A general rule to keep in mind is this: if something smells like plastic, don’t use it.
- Soak the item in hot water. If it has a plastic smell after that, don’t use it. If the plastic feels slimy in the water or goes brittle when dried out again, don’t use it.
Really, any plastic decoration that isn’t made for aquariums should be viewed as risky. It might be fine – but then again, it might not.
Most smooth glass items are fine. There’s a couple of exceptions I’m aware of:
- Lead crystal – because… it contains lead.
- Antique glass – because lead was very commonly an ingredient in glass decorations.
- Painted glass – glass paint can be extremely toxic.
- Unsealed mirrors – that stuff on the back of a mirror which makes the glass reflective is often really toxic. Don’t stick mirrors not marked specifically for aquarium use in your tank.
Basically, any item that cannot be used for human food or drink (like raku pottery, which contains a LOT of heavy metals such as copper and lead) should never be used in your fish tank.
Unfired pottery like terra cotta is very porous, so should never be used if there’s a chance it has been exposed to detergents, garden sprays and other toxic chemicals, or used for plants (in case of fertilisers).
Brand new terra cotta can affect ph levels, so it’s best to ‘age’ your pots a while by soaking them in a bucket of tap water before use for at least 24 hours, and then testing the ph.
Avoid ALL metallic glazes.
They can be horribly toxic.
Bright red, blue and yellow glazes on non-food grade ornaments that are not specifically made for aquariums MAY be very toxic, and MAY leach poisons into the water. I’d personally not risk it.
The subject of wood in tanks really needs a whole thread to itself, and there’s been a lot of good advice given on this forum already, so I’ll keep it brief:
Wood marked for aquarium use at the LFS is often quite expensive – but this is because it has been aged, pre-soaked, cleaned and treated to make it safe for use. Common woods for sale at LFS’s are Mopani, Bogwood, Redmoor root, Mangrove root and clean driftwood.
Be very careful about using wood you have found outside.
Wood is very porous and absorbs environmental toxins and poison sprays easily, so don’t use wood found in the garden unless you are very sure nobody’s ever used herbicides or pesticides on that land or nearby as sprays can drift a long way.
Some wood contains sap that can be extremely toxic in general, especially when the wood is green. To be safe, never add any unidentified wood to your tank.
Never use green wood
(cut less than a year, or better, two years ago) in your tank – some wood that is safe when completely dry is not safe when green, or even for several years after it’s been cut. It may contain sugars and starches that will upset your tank’s chemistry, or toxic sap.
Shells and Coral
These aren’t recommended for use in freshwater tanks, because they contain calcium which dissolves in water and can dramatically increase the ph and hardness levels.
Not every rock is safe for aquarium use
, especially in water with low ph
Aside from the danger of sharp or jagged edges and rough surfaces, some rocks contain high levels of toxic minerals and elements (like lead and arsenic) that can leach out into the water and will quickly poison your fish.
Low ph makes water soft and acidic, which frees otherwise inert (safely 'locked up') minerals and elements from the rock so they leach into the water, making it toxic.
Some minerals (like fluorite and calcite) will dissolve in water and affect your water chemistry.
Speaking VERY generally, the safest rocks:
- Are extremely hard
and will not go soft even after a month of being soaked in water, and do not shed a lot of particles or scratch badly when scrubbed with a scourer.
- Do NOT have any metallic particles
in them, something you sometimes can’t tell without a microscope…
Generally regarded as safe:
- Clear and white quartz (don’t use spiky crystal clusters, for obvious reasons, and avoid any quartz that contains glittery fool’s gold, which can be very toxic).
- Igneous rocks (Obsidian, Granite & Gneiss – watch out for sharp bits, though)
- Petrified wood
- Very hard, smooth, water-worn ‘river rocks’ (must be boiled before use, don’t oven-heat though as some will explode in dry heat).
Cautionary, best avoided:
- Shale and slate can contain pyrites (poisonous metals) and sometimes fossil oil residue. I’d avoid collecting them for aquarium use, just in case.
- Limestone, dolomite, marble, sandstone, siltstone, calcite, fluorite and other ‘soft’ and ‘brittle’ rocks contain minerals that will dissolve in freshwater tanks and quite seriously mess up your water chemistry.
- Brightly coloured minerals and crystals you might find at a new age store, gift store or market - like Turquoise, Malachite and Azurite, which have very high levels of copper and/or aluminium, and Lapis lazuli which very often contains toxic pyrites.
AVOID at all costs:
- Any ore (metal bearing rock), and any metallic or ‘glittery’ minerals - these will almost certainly poison your fish.