He could of been ill from when you got him, since it's only been there for a week. Not uncommon for many pet store fish to die within a couple weeks of getting them.
Plecos need a ton to eat - make sure they get some wood and meat in their diet.. algae wafers are fine, but not a staple meal. They will even eat on live plants, usually you will want to supplement their diet with veggies such as cucumber, zucchini, spinach and romaine lettuce. They also love frozen foods such as brine shrimp and blood worms, freeze dried, etc. If you have the ones I'm thinking, they reach about 5 inches and do best in tanks 20g+. They produce a ton of waste, so keep up on the water changes.
Last edited by Myates; 04-29-2013 at 12:30 PM.