Its a matter of how well off they are. I've only had issues with plecos that I have had less than a year, the older ones are fine. There's also a huge problem with the naming of common plecos, not all are the same thing. Its not so much I had a weird pleco, it was mostly because he was starved at the store and I was stupid enough to think he would be fine with the koi and goldfish for a while. What ended up happening was all the algae disappeared overnight and a few of my fish were harassed, one in particular was harassed so much his slime coat and scales never came back and he didn't make it in the long run. I've had friends and accounts on forums tell me similiar stories of a pleco that they got at the store that finished up all the algae and then went on to the fish because they were simply starving. Not saying that all are like this but there is a chance and its better to be safe than sorry.