My friend was talking to me about how he got two turtles from our chemistry teacher, and how he had them in a 100g tank. I was like OK cool, and then he stated taking about how he added goldfish. I started telling him about how goldfish and turtles do NOT mix, but he said that after they ate 4/12 of the fish, they play with the last 8. I started telling him about how they need a filter and he insisted that he knows best because his fish have always lived 2-3 years.
Basically I'm asking:
How many things is he doing wrong?
What exactly do goldfish need in order to be as healthy as possible?