So this quest (aquarium zoo, npc kenta)is repeatable, after collecting 5 smaples from boxes in auqa road, you can re-accept quest, and immediately finished it, and do this infinite loop again and again. However by doing so the npc doesn't reward u any exp or the supposed 10 (or 20 maybe) pet closeness, so there will not be any abuse of the bug, and it doesnt boost Quest Specialist Challenge cuz it is repeatable quest. I also found a bug in quest The lost Seed's following quest,(korea folk town, swallow npc) which asks u to plant the seed on one of the brothers' roof top, it used to be as nothing would happen after ur droping, now it has been fixed. But it seems if you drop the seed at the poor brother's roof, the gourd that summoned by seed wont drop the item required to clear the quest, only on the rich brother, Nolbu's roof will.
The repeatable version of Kenta's Advice is supposed to work like this, however your pet never actually becomes faster because of it, which is a known bug. The issue with the quest "Opening Hongbu's Gord" will be fixed in the upcoming update.
Thanks Andreas! Maybe what I remembered is wrong then, I once checked kenta's quest guide on a Chinese web, and it told me the reward is pet closeness, and I didnt go throught what kenta says in game.
It does reward you with closeness, and pet speed, just not when you do the repeatable version, that version is only to make your other pets move faster. (Or rather it should reward you with pet speed if it wasn't bugged for now)