Why did my pleco die?

I had a plecostomos for 18 months, he was six inches long and seemed perfectly healthy. He kept my 44 gallon aquarium nice and clean. One day i came home from work, and my four oversized goldfish, had EATEN him. I was devastated, and after months and months of cleaning the tank myself, (we are isolated and no fish store anymore) I bought 6 small plecos again and my fish killed nearly all of them... or did they die on their own. They all had good color and were eating good. Any suggestions, my tank is so green and its deep and I would prefer to get something to eat the algae.