Answer:
Koi: There is a Japanese/Nihongo myth that says if a koi fish is capable of climbing the walls of a waterfall, a point called "Dragon Gate of Yellow River" that it would be transformed into a dragon. So, in essence, the Koi fish tattoo is supposed to symbolize advertisement and aspiration. The koi also represents strength, good fortune, and eternal wealth.
Dragons: They are similar to a ying/yang symbol. They represent the elements (Water, Fire, etc), the directions (East, wast, etc), and the Sky and Underworld. They are a balance that leaves them almost neutral, so they are neither bad nor good. Though, they are often seen has "bad tempered" though is still associate with neither bad nor good.
They are highly associated with Buddhism.