My fish all died in my aquarium. Should I get new gravel, artificial plants, ornaments, and new rocks?

The problem is that no one knows exactly why your fish died. Some reasons might be:

-Tank just set up and water too new. Fish tanks need to adjust for a few weeks. Although you can add some of the bacteria available at the fish store, that may not do the full job. A fish tank depends on a balance in the water, and without this the fish will die. They may appear to be suffocating, near the surface, gasping, etc. Changing gravel, ornaments, etc, will not change this.  Water directly out of the faucet is not healthy for fish, and will kill them. It needs to "age" and also be treated with anti-chlorine drops.

-Anti-chlorine drops: Always use this with new water and for water changes.

-Changing water too often: Do not change water too much. This upsets the balance if you already have an established tank. The filter should do the job, with changes of only about 10% each week. This applies to an established tank only.

-Ornaments or plants not made for a fish tank: If you placed things in the tank that were not made for a fish tank, this could kill the fish. If the items were all made for fish tanks, then it is not likely you need to wash them.

-Too many fish in tank: If your tank was terribly overstocked, this might be to blame, but it might be expected that a couple fish may have survived. Also, sometimes goldfish can pollute the tank if there are too many, because they create a lot of ammonia.

-Filter inadequate: Be sure the filter is appropriate for the size of the tank.

Talk to your aquarium store. Set the tank up properly. Use proper items from a fish store inside the tank. Use a couple test fish for a couple weeks before restocking, AFTER you have set the tank up, let it aged, etc. Rushing this results in fish deaths.

Always siphon the bottom when cleaning with a gravel siphon.

I would sterilize the tank just in case of bacteria or viruses. I soak everything but the gravel in a 1 to 20 parts plain bleach and water which will kill anything that may be there. Rinse well and use dechlorinator until there is no bleach smell and dry in the sun. I boil the gravel in a pot on the stove or if it is a large amount I rinse it and lay it out in a large plastic pan to dry in the sun and rinse it again in tapwater before using it.

If you lost all of your fish rapidly it may have been ick. Were they darting or scraping themselves on rocks, plants or gravel? This can be easily cured by raising the water temperature to 86 degrees for 2 days, or use ick medication.

 Most likely it may be from new tank syndrome which is usually ammonia burn from too many fish in new water without a biofilter for the good bacteria that balance your tank. This plus overfeeding (#1 fish killer) will cause rapid death. You can learn about the natural cycle of aquariums from a book or many videos on Youtube. Father Fish is a great source to learn from.