Not all things people put in a tank to ornament it are safe for all tanks; not even all of those sold specifically for aquarium use. Decorations can cause problems in several ways: they can be sharp or brittle, they can alter the pH of the tank, or they can introduce toxic chemicals into the water.
Glass gravel is an example of a tank furnishing which is potentially sharp. While such gravel may be completely irrelevant to fishes that spend all their time in Mid-water or near the surface, glass gravel is not recommended for use with fish that burrow or dig in the gravel, such as corydoras catfishes, because it can cut their barbels. Open wounds are an invitation to infections which could potentially kill your fish. Some china aquarium ornaments may also be sharp. Check your ornaments before you put them into the tank, and if they break check the broken edges. Sandpaper can be used to smooth sharp edges.
There are several types of tank furnishings that are likely to alter the PH of the tank. In some cases this may be good, it may be exactly why you bought them in the first place, but it is necessary to know what effect they have and keep an eye on it. Driftwood and peat adds tannins to the water making it it softer and more acidic. This may add a slight yellow colour to the water, and is useful for keeping and breeding acid water fishes such as many tetras. However, it is a good idea to watch the PH. I live in an acid water area and I love to use driftwood in my tanks. I keep acid water fishes, but I find that the PH has a tendency to drop very low when using driftwood, to the point where I sometimes have to add an agent to make the water more alkaline.
Shells, crushed coral and soluble alkaline rocks such as limestone or calcite increase water hardness and pH. If you are trying to keep Rift Lake Cichlids or marine fishes, this may be exactly what you want, but again, the pH must be watched and prevented from becoming too extreme for the fish you are keeping.
Certain materials are toxic to fish and can leach into the water. Copper is toxic to fish so pennies and ornaments made out of copper are not safe for your tank. Lead is also toxic and can leach into water. For this reason, objects made out of lead are a bad idea, and objects with a lead based paint could also potentially harm your fish. Decorations bought from an aquarium store should hopefully be free of major toxins, but ornaments designed for other purposes may or may not be. If in doubt, don’t put it in your tank.