I find this strange too. They should be able to handle anywhere from 32-80 degree's just by their native range in Europe. I know the cooler water ones in Germany and Russia for example do not grow as big because the cooler water does not put their metabolism and appetite into overdrive like it does in the Ebro. It has me thinking that if these are farmed fish from Spain they have adapted to that 76-78 range so much they cannot handle anything cooler or warmer. If I had to guess 76-78 degree's would probably be the average water temp in parts of Spain. The only other thing I can think of is that is what temp the fish was raised in from an egg and ot just cannot adapt as well to any other temp.
