What I have calculated before the event ends we will get total of 3 x Iridescent shellfish.

I got my first one this thursday 3/8

Event ends 23/8

That's 3 total

Am I missing something ?

Speaking of Dorio, and maybe something that's not right, I completed all the quests (I think? Dorio is no longer offering any), and still do not have the knowledge.  Something is off with Dorio, or are things just not explained clearly, or ?

No matter how i calculate late it, I only get 3 shells in 3 weeks.

So yes, you can't get the fish tank.

Or maybe someone can tell me how to get the 4th shell.

Your missing info is in the event notes. :)
"* This quest will remain available after the event ends."
Source:  https://www.naeu.playblackdesert.com/en-US/News/Detail?groupContentNo=5642&countryType=en-US

This sentence wasn't there, when i wrote my comment!

But I still don't get how you can get the fourth shell, because the fourth week, to get the last shell, would start on Aug. 24th 0:00 UTC and the palace closes on Aug. 23rd after the maintenance!

I didn't see the sentence at first either.
I'm guessing the sea palace will stay there permanently, or the otter will be moved somewhere else.

I am fairly sure the palace will remain after as it is has a book icon in the new tab and is available on all character like the single tales of LoML quests throughout the land. Only thing I expect is the velia connection to be removed.