Hollow pond or aquarium ornaments come in all shapes and sizes, from sunken ships, to model houses, chinese temples, slightly open treasure chests, divers helmets, artificial wood, rocky caves, fake skulls, sea shells, or purely decorative. Unfortunately they are a bad idea. The problem is that the water inside the ornament does not circulate. This means it is low in oxygen and high in bacteria. In other words it is stagnant, and the ornament has become a disease factory. Bacteria escape and enter the main body of water, making fish sick and killing them. Without knowing what is causing the problem, an otherwise diligent fishkeeper can end up very frustrated trying to find out the cause of the infections. Thankfully there is an easy solution that doesn’t mean removing every hollow ornament. The silicone sealant that is used to hold aquarium glass together can be injected into the eye sockets of a fake skull, the opening of a sea shell, the windows and doors on model houses, and so on. This means that the ornaments are no longer hollow (even though they look it), and interesting or themed aquariums don’t need to lose their character to be safe for fish.

(Source: happy-goldfish.com)