How to get this quest
You get this quest from Roland in the Knights Keep after completing Rampaging Golem.
Finding the lair
First, speak to the elixir alchemist. His shop is located north of Verdeaux's workshop. You can now optionally talk to Trademond's alchemist about Tofana's third theorem (this unlocks Something is Fishy later), but your next objective is the Vagrant next to Verdeaux's cathedral. To find the lair, you've got 3 options:
Fightning the poisoners
The poisoners' lair is located south of the cathedral. The entrance is on the building's west side.
Once you enter the lair, a fight happens. The poisoners use Defensive Stance and their normal attacks can poison you. They have low HP, so magic or Lightning Sword can deal quickly with them.
After the fight, you will receive the quest Something is Fishy if you meet one of the earlier conditions.
Reporting to Roland
To complete the quest, the last step is to report to Roland. If you have Something is Fishy active, note that reporting to Roland before confronting the alchemist is a Vice action (5 Vice requirement).