Typically they are unaffected by it and I have caught mine nibbling too. Snails all over mine too and they don't seem to have any problem with it. My fish seem to like charging into the current and then peelingf off at the last second. It seems to be kind of a game to them. The sailfin will do this repeatedly.
When I first saw the nibbling late last week, the thought occured to me that maybe that is why I have had the rattling problems. But I figure that is the first time I had seen it so I have concluded that is not the case. Since getting the new washers and tightly securing the power cords, I have had virtually no rattling problems. They are humming along and I just wish I could control them now. I am doing it by hand each morning and night...