Read a lot of books, and listen to the experts!
Niger Trigger - 6"
Snowflake Eel - 8"
 Cleaner Shrimp - 2"
Coral Banded Shrimp - 2"
Tiger-Striped Star - 6" diameter
Sand-Sifting Star - 4" diameter
Various hermits and snails - 0.75-2"
Life's an ocean, dive it.
I'm 14 years old and love the saltwater hobby! Not much of a freshwater person, but I do breed Orange Sunshine Guppies.
When you see signs of stress in your fish, you can then take steps to identify the source of that stress and then to resolve it before it becomes a major issue.
If you're looking for a unique way to stock your new tank, give freshwater shrimp a try.
If your fish is suffering, you may want to consider euthanasia as an option to humanely end his pain.