Why did my pleco die so fast?

I bought him from my LPS and then let his bag sit it the water for over an hour. After a let him out and he looked. He did have some tiny dots all over him that were very small and yellow that blended whith him because he was albino. After he would hang around my heater. Then after a day I notice his tail fin was shredded. And after this 2 times in a row he swam right in to the filter and was getting sucked um and I had to get him put with my net. I think the filter pulled on his fin and wrecked it. After the next day I woke up to find him dead. And yes I'm gonna go and get a refund and get another fish but how can I stop it from dying. Could the spots of been itch. They were on his body and fins. Or did he die of swimming into the filter and getting hurt. It does look like he has a red scar or mark on his tail that he didn't have befor. Also, the water temperature is 75 fareinheight. But I don't know the pH but my guppies have been living in there for over atleast a year or longer with 50% water changes every few weeks