I heard that goldfish can eat a veriety of fruits and veggies, such as oranges, zuccini, broccoli, lettuce, spinach, cucmumber, strawberry, pumpkin, banana, peas, etc.
1. I need to know how to prepare each of these foods to ensure it does not damage my fish as well as how long to leave each in the tank (i usually do for 5 or so minutes).
2. I heard from various sources that a daily supplement of oranges (vitamin C) is quite beneficial. Is this true?

Thear are a lot of food, even so, in my experience i would prefer use only a comercial well balanced formula. Some trades like hikari, nutrafin, sera, tetra, wardley and otto provides formulas specially formulated for a goldfish. Some times i offer to my "goldies" live or frozen animal food like artemia, tubifex and bloodworm.

Some time ago i was interested in other kind of food; but i observed that the polution in the ather was quickly. I´m ok and my fishes too (i think) with special comercial forlmulas