Gravel is very important in a fish tank, and you should have about a pound of it for each gallon of water. The gravel not only makes for a more natural environment, but gives the good microbes a place to thrive in your tank. These microbes help to manage waste and keep your water healthy. If you don’t wish to have gravel, you can use sand or small pebbles such as substrate for the bottom of your tank.
As far as using stones for decoration, that’s an aesthetic decision and totally up to you. However, I will caution you to only choose stones and rocks that are safe for aquarium use. One easy way to be sure of this is to only use stones you’ve purchased from a live fish store or pet store, and that are specifically intended for use in aquariums.
You can also hunt for stones outdoors, but beware that this can be potentially harmful to your fish. Some stones can have minerals or dangerous metals in them that will leach into the water. They may even have fungus, parasites or pesticides. So, if you do choose rocks from your yard or garden, be sure to do some research and find out what they are and if they are okay for aquarium use.
I have used rocks from the outdoors in my tanks before, but I wasn’t very comfortable with it. My general rule of thumb is to only put things in fish tanks that are made to go in fish tanks.