Right! Yeah it's easier to sell fish that look nice and have good form even if they aren't being shown. But it just makes for a healthier fish too
As long as you don't breed him to a DT or another HM he should be fine. If you want to keep the dragon scaling you should breed him to another dragon girl as well, same goes for color. If you want to keep that color then breed to a girl with a deep blue dragon scaling, otherwise you can mix and match to see what you get
Of course you won't get all Dragons or the same color if you breed two like fish unless the sister he has has great form because you can end up with grandparent fish genes and stuff.
Breeding and genetics are so intriguing