Metals:
* Corrosion: Most metals will corrode in the wet environment of a fish tank, releasing potentially toxic substances into the water. This can harm your fish.
* Sharp edges: Metal objects can have sharp edges that can injure your fish.
* Temperature changes: Metal can drastically change temperature, potentially stressing or harming your fish.
Wood:
* Rotting: Wood will rot in a wet environment, releasing harmful bacteria and chemicals into the water.
* Release of tannins: Some woods release tannins into the water, which can stain the water and lower its pH, potentially stressing your fish.
* Harboring bacteria: Wood can harbor harmful bacteria and parasites that can infect your fish.
Exceptions:
* Specifically designed materials: There are some metal and wood products specifically designed for aquariums. These materials are treated to prevent corrosion, rotting, and leaching harmful substances.
* Driftwood: Driftwood is a popular addition to aquariums as it provides natural hiding spots for fish. However, you should always soak driftwood thoroughly before adding it to your tank to remove any harmful substances.
Alternatives:
* Plastic or glass: These materials are safe and durable for use in aquariums.
* Aquarium-safe decorations: There are a wide variety of aquarium-safe decorations available, such as rocks, caves, and plants.
Before adding anything to your fish tank, it's always best to research the material and make sure it's safe for aquatic animals.