Because some people get stuck or (understandably) don't have time for this I'm putting out this small walkthrough.

This ARG isn't a very elaborate one. It's meant to be time-manageable. If you spend time reading all the stuff, then it gets longer, but you could basically just breeze through it and get the reward at the end.

So here it is...

The opening page has a bunch of options. These are all just silly tangents where you can click silly things and get a laugh.
The thing starts when you choose "ENTER SITE"

This is the main mystery area!
So there's lots to poke around in here.
The premise is to find out what happened... Where did THE Fish go? Why did it disappear? Where did all the other inhabitants of this space vanish to?
You discover all that by reading and exploring everything here.
There's a lot of content. I won't spoil all that, so do dig in, but if you don't have time for that...

...To progress PAST this part, you need the NAME and PASSWORD to unlock the ESCAPE Site option.

To get the password you have to feed the virtual vermin (named Verm) parts of the site. You do that by browsing.
You knock down parts of the site. One they land you can let him go get them, or drag/drop the items to feed him manually.
Once you've knocked down enough information you are thanked for saving him (he was starving).

The password for sweetEscape (ESCAPE Site) is "goldfishy". Nobody could have guessed that!

Beyond this point Verm will continue to eat parts of the site that you knock down. He is forever hungry.

To get the USER NAME for sweetEscape you have to help out the fishy that's at the top right.
Chatting with her reveals that she has a BIRDY friend that she's really worried about and has been searching for (for a very long time).
BIRDY signed up to be a sprite in this really bad browser game, and is now subjected to a mundane existence.
She tells you that she looked everywhere except for the Fish Demos. Talking to her unlocks the Fish Demos page.
She tells you to go look for BIRDY. Once you find them talk to her again...

The Fish Demos page has a bunch of stuff to mull around in. There are a few games.
BIRDY is trapped in the Browser Game, which you can visit by clicking on "Investigate the browser game."

Floppy Bird is indeed a torturous existence, but you are glad to know that BIRDY is fine (all things considered). They're fulfilling the role of a sprite in this game, and there seems to be nothing to worry about... so go back to GOLDFISHY and tell her that...

She will be happy to hear that and will give you the NAME for sweetEscape...

So the USER NAME is: gogopowerfishy
and the password is: goldfishy

You now have everything you need in order to escape this site and pursue what it is that you are looking for.

Once you enter ESCAPE with the proper username and password, you find yourself in an open directory.
There's lots to mull around in here, and I suggest you explore just a bit.
Some cute things to laugh at, like the artist in residence that's located south.
There's also an option to stare into the void if you go west.
North has quite a few things to look at like the bat in the cave, and the beautiful view in the hills (if you get past the electric wizard).

After all that, what you do want, is to go visit the East Village because that's where fish passed through (a long time ago)...

Obviously something went wrong here and you have to help the inhabitants...

The inhabitants are being attacked by a virtual vermin that got into the directory and is now eating everything they love.
Click on them and they will ask you to defeat the vermin...

Defeating the monster is easy (because you're a power user, so there's nothing holding you back).

Once defeated, the villagers tell you about the disappearance of the fish (the one that you are looking for). There's a lengthy dialogue explaining how Fish disappeared a long time ago (they helped), and how Fish never got the license key from them because their champion villager never returned. They assume their champion failed the mission, so they ask you to deliver the license key instead. They are very confident that you will succeed...

So you have part of a license key. It's:
HOT-BIG-GIGABYTES-123
But you're not totally sure what it unlocks yet...

The villagers show you the way, and slowly vanish, leaving just a door in your wake.

Entering this door leads to a long journey through nothingness.
It's fairly poetic at this point, drawing heavily on the existentialism of the digital void.

Once you embrace the emptiness (an option you are given if you keep walking forward), you finally reach the end of your journey.
Here you find the bones of the champion villager, that clearly failed their mission.

On them is a note giving you the final bits of information you need in order to find THE Fish...

The note links to a secret itch.io page from THAT developer, and also gives you the rest of the license information. You now have everything you need in order to unlock and possibly save THE Fish.

Note that the license key is:
HOT-BIG-GIGABYTES-123
And the smaller (inferior key) is:
TINY-KEY-123

You need both. You're not certain why yet, but it's good to keep notes...

Following the link to THAT developer's itch page, you finally find Fish. It's downloadable...

Go on and download it!!

Run fish and enter the appropriate key.
You'll finally get all your answers...

So you find THE Fish, but you're probably a decade too late.
There's a small set of story that follows. Here, I'll spoil it...

Pay your respects... You do that and it invites you to explore...

Taking a closer look reveals something near the skeletons head. It's a bundle of processes that haven't completely deteriorated yet. It looks like quite the abnormality, and (to your surprise) it seems like it could still run.

So you decide to access it...

Obviously this is what the second key was for (the inferior key)...

You find a friend!

Your friend is a tiny fish can live on your desktop. It's small and unintrusive.
She will sing to you, get surprised by things happening, hide (so you have to find her), and go belly up... She's very sweet and cute.

That's the ending. You make a virtual friend!