Use a water test kit to test the water. If you see spikes in ammonia or nitrite, then the decomposing fish has taxed the your bio-filtration and you WILL need to wait until these levels reach 0 again.
By saying your "only fish died" do you mean that you only had 1 fish, and it died, or that the all the fish in your tank died?
What is the cause of this fish or fishes death? You need to know this, if possible, before adding more creatures back into your tank.