The colour and overall body shape are the first characters to improve as a result of selective breeding, and the large tail is the hardest character to achieve. It is not necessarily the rule that good fish produce good young, which can be a puzzle, and improved characters, if not carefully maintained, will deteriorate – in fact the fish, if left to themselves in large numbers, would steadily revert to wild type. Some find that improvements in the tail lead to deepening of the body, and it is a skill to achieve a good balance of characteristics in accordance with the standard.