Ahhh...... Decorating the tank: One of aquariums' great pleasures! But pleasures like these always come with a price: A really high price sometimes!
I think it's better to have a "natural" theme for your aquarium(but I'm not forcing!) because:
1. Nature is everywhere! (except for cities, of course)
2. Your fish will love it and they will feel relaxed because they feel home(except for the big ugly faces watching them)
3. You too will feel relaxed!
Yes, get some rocks outside but be sure to clean them first because they might have some dangerous chemicals or bad organisms in them!

And plants: They make your tank livelier! Although they're a little expensive in stores, you can get mosses outside that you can put into your tank(attach them to rocks or ornaments) and they will grow beautifully! They're also one of the easiest to take care of and they don't need much light!
Well, there's my advice! Happy decorating!
