- Plants: Plants are one of the most important decorations that every aquarium must have. Please read this to find out whether your aquarium is suitable for plants or not and if you can keep plants, then which plants suit your aquarium best
- Rocks: If you really want rocks or shells in your tank, buy some special shells made for freshwater tanks from your local Pet Shop. Never pickup seashells and put them into your tank! It is a certain and slow death for your fish. The calcium released by shells and coral can do real damage to the fishes.
- Caves and Rockeries: These are a must! Certain fish are more nocturnal than others and some just need the protection. Besides it really adds realism to your tank's environment. Always pick some up at the fish store and not from your front yard or by the beach. It may be a bit more expensive at first but will save you money in not having to replace the dead fish all the time.
- Wood and other things: It is safe to place items in your tank as long as they are inert, meaning they won't release (leach) chemicals into the water. Most plastics are inert, as are glass and ceramic.
Wood may leach substances into the water, changing the pH in a possibly inappropriate manner. Driftwood often leaches tannins and other humic acids into the water (much like peat moss), possibly softening it and lowering its pH. The water may also obtain a yellowish tea-colored tint. The tint is not harmful and can be removed by filtering the water through activated charcoal.
If you use wood that you've found yourself (e.g. in woods or a lake), boil it first to kill any pathogens. Boiling it (long enough) will also make it sink in a tank.