Unsafe Materials and Decor That Leach into Aquarium Water
Uncured silicone, fungicide sealants, painted ornaments, treated wood and reactive rock can poison a tank. Learn how to vet new decor and what to do if something leaches.
Not everything that fits in a tank is safe in a tank. Many materials slowly leach harmful substances into the water or change its chemistry. Vetting decor and equipment before it goes in is far easier than diagnosing a mystery poisoning afterwards.
Silicone and glues
Silicone sealant is widely used to build and seal glass aquariums because fully cured silicone is inert, but not all silicone is aquarium-safe. Crucially, mildew- or mould-resistant kitchen and bathroom sealants contain added fungicides/biocides that are toxic to fish, so they must not be used. Choose a sealant that is 100% silicone with no mould inhibitors, and let it cure fully (commonly around 24 hours, or longer per the product) before water contact.
Decor, wood and plastics
- Painted, varnished or treated ornaments, and items not rated food- or aquarium-safe
- Aromatic woods, which contain toxic phenols, and pine or yew branches, which leach toxic sap
- Plastics and resins not rated aquarium-safe; most plastics are inert, but recycled plastics may not be pure enough
- Metal items and hardware (see the heavy-metals guide)
- Anything that has been washed with soap
Rock and shells
Rock should be inert: free of metallic seams and roughly pH-neutral. Carbonate rocks dissolve slowly and raise hardness and pH, which is fine for hard-water tanks but unwanted in a soft-water setup; shells and coral do the same. Test a rock by dripping a little acid such as vinegar on it: fizzing means it contains carbonates that will raise the pH.
How to vet new decor
- Prefer items explicitly labelled aquarium-safe and made of inert materials.
- Cure silicone and glues fully and rinse everything in plain water (never soap) before adding it.
- Do the vinegar test on unknown rock and decide if a pH/hardness rise is acceptable.
- Soak and monitor questionable items in a separate container, watching for film, odour or parameter changes.