The Fishtank Quest "Timeless Memento" and "Recall the Memories" requires the "The Tale of Shim Cheong" knowledge.
These knowledge is from the Shim Cheon Questline obtained by the NPC "Shim the Blind" - The npc is located on the cliff before you enter the water palast. Many new player that going for the fishtank get confused why they just can't continue in the Fishtank Quest after already done 3-4 Quest because of the missing knowledge. I think a requirement should be there directly on the first Quests or just remove the Knowledge needed. People go into the Water Palast - finishing the first Quests and then need to leave to also accept the quest from "Shim the Blind" seems a really unnecessary step in the middle of the Fishtank Quest.

So moving the requirement to the first quest of the Fishtank or completly remove it would be more friendly to the newer player.