Glimmerfish Before It's Gone

The Giant Glimmerfish is a gigantic neutral fish mob that starts at the top of the river system & gradually swims downstream.

If you get offered the quest just fly along the river until you find it - despite being gigantic the nature of the slow progress downstream means you can get proximity offered the quest while you're well out of sight of the fish.

You attack the fish, scales drop off, and you loot the scales.

Once enough have been looted the event is over. No need to keep attacking it constantly, no need to hand the scales in to any NPCs - just hit, loot & repeat.

Seems bugged (at least for some servers.) The objective pops up on your screen when in the vicinity of the river, but going up and down it shows no giant glimmerfish.

The Giant Glimmerfish spawns at 46.42, 86.77 in Zaralek Cavern,

It follows the water path dropping down from all waterfalls until it reaches lava in 40.83, 58.31,

Once it reaches lava, it despawns,

To collect scales you have to hit it (a single hit is just enough) and collect all dropped scales, you can track the progress of the quest by hovering over Giant Glimmerfish and it will display % of collected scales above its head.

Quest is provided when you fly over it.