Aquarium size and shape.
Substrate type and depth.
Type and size of filtration.
All of the fish and their sizes.
Live plant types and quantity.
How long the tank has been setup and is it cycled?(Cycling is covered in FAQs).
Lighting type and time on/off.
Feeding, food types, quantities, and frequency.
Water change schedule: how much and how often, when was your last water change?
Any symptoms, odd behaviors, or visual observations.
Any injuries the fish may have sustained and how.
Anything else you can think of that might help us help you. The more we know of your system the better we will be able to assist you so include as much information as possible in your post.