Why is my Pleco sucking on other fish in my tank?

This could differ depending on the breed of Pleco. However, some Pleco will resort to "feeding" on fish when they are not having their nutritional needs met. Some Pleco require protein or meat in their diet. When they are starved of proteins, they will seek this from other inhabitants via their slim coat. Once a Pleco gets a taste of slime coat, it is difficult to transition them back. However, try feeding your Pleco at night time just before the lights go out. It may be that other inhabitants in the tank are eating the food source provided before the Pleco gets its fair share.