Decorating your tank may cost you money but would give you experience points in return. If you really want to succeed in this game and get the most out of the experience that you can, you'd be wise to pay closer attention to your FishVille decorations. They'll wind up playing a much larger role in your overall success than you think.
You only get this experience if you buy the item yourself. Relying on the gifts you receive from your friends for all of your FishVille decorations will save you some cash, but it won't allow you to rack up these bonus experience points.
Of course, you can't just go throwing money around either. The last thing you want to do is go out and spend all of your coins on decorations only to realize that you have none left to buy new fish with. That's why you should take a few moments when you first start playing the game to look through the decorations table below.