Oh, sorry. Just noticed your new post on fantail. I think mine may be a cross. Its body is not so short and round like the fish on your pic but it looks more like a single tail's body. And the tail is a bit different from your fish's. She is still a beautiful 'mongrel'... By the way, do they slow down as they get older? I have seen some fish that just gracefully float in their tank while mine goes round in circles like mad
I would say they calm down as they get older similar to us, my 2 big ones don't do much now and they are 4 years old "that is except when I come into the room or go near their tank" then they swim upto the front of the tank expecting to be fed, if it is feeding time they are more active as they swim or grub about for food but at other times they just rest on the graval or float still like in the water, in my other tank I have 4 young fancies 2 orandas a black moor and a golden telescope and they swim about a lot.
I think as they get bigger, they become more cumbersome and so such effecient swimmers, hense the tip, never mix double tails and singles, they bodies are very round and the fins long and flowing, not exactly streamline. They float and cruise i used to think my moor was dead, if was so inactive. Just conserving energy i think.