If you are going to decorate a fish tank it is a really good idea to NOT use any found items. By them at a reputable shop or you could poison your fish.
As a general rule, fish like cover. Fish that are from shallow ponds or lakes, or slow-moving rivers prefer “soft cover,” which is primarily plants, but includes other fine or lush forms of decor. Fish from rapid running waters or from deep, open areas tend to prefer “hard cover,” which is generally rocks, logs, and other large, solid objects.
For most fish, covering the tank with decorations over 50% to 75% of its space is appropriate. This means that a great deal of your aquarium will be taken up with decorating material. However, this also means that your fish feel comfortable. Remember that fish like tanks that have lots of decorative cover.
In a freshwater tank it is also a good idea to avoid real shells and coral skeletons, as well as limestone, as these materials will dissolve slowly in the aquarium, increasing the hardness and pH of the water.
It might seem obvious but please remember to put larger plants and decorations at the rear of the tank – this way you can view the fish whenever you want without obstructions. You can deliberately use taller decorations and plants to conceal the workings of the fish tank such as pumps, air tubes and wires.
Use groupings of the same type of plants and offset them with groupings of plants with a different color or texture to create a more attractive space. You can also use smaller plants or rocks to hide the edges of castles, volcanoes, water wheels, or other resin, ceramic, or plastic decorations to make them look more natural in the tank. Take two decorations that are your favorite and make them the focus of your plant.
Do not hesitate to give your fish a background to swim against. background provides a dark, or at least solid, wall for the fish, which can help make them more comfortable. A background will also help hide things that could otherwise detract from the appearance of your aquarium.
Decorate your fish tank with decorations that you like – provided that they are not toxic to the fish.” In most cases, the fish don’t care what your enchanted castle looks like! How natural you want it to look is truly up to you!