Your save file is located in C:\Users\YourName\AppData\Roaming\Godot\app_userdata\Chillquarium\
Copy the entire V7 folder to somewhere else on your computer where you will remember it. This is your backup in case you make a change you can't but want to reverse in game.

In the event that you want to restore to this backup, simply paste it back into the save folder and replace all files.

Next we are going to launch Chillquarium and set up the fish we would like to edit. In my example, I am upgrading the color of a single fish to make things easy. Keep note of the tank that these fish are in, it is important to know a bit later.
In my freshwater 2 tank, I have 1 Arapaima that I want to make gold. I highly recommend giving the fish a specific nickname that you will remember as it makes it 100x easier to find in the files later. It's not a very big deal if you have 1 fish in the tank, but if you want to edit fish from large capacity tanks, it gets confusing quick.

Now be sure to save and exit the game, and go back to the save file location on your computer mentioned before.
In the V7 folder, seen above, you will find a number of .SAVE files depending on the amount of tanks you've accrued at this point in your playthrough.

the files are numbered in an array fashion, each file corresponding to one of your tanks. Arrays count up from 0, so tank_0_objects.save is actually your freshwater 1 tank (meaning tank_1_objects.save is freshwater 2 tank in game etc etc.)

Locate the tank that you want to edit, in my example case, we want to edit freshwater 2, so I want tank_1_objects.save.

EDIT: as of July 8th, 2024, I was unable to get this specific website to work. There are a million other ways to edit a .save file though, so I'm not going to write a new example. This time around, I opened the .save files in Notepad++ instead.
Now we are going to open these save files and edit them. Whatever preferred method allows you to edit the text is fine, I personally found and used this website to edit mine: https://www.saveeditonline.com/


Simply upload the file to the website and it will allow you to edit and download the file.
Now to find the fish I want to edit, I simply control + F and keyword search the nickname, in this case, MAKEMEGOLD

Now here in these square brackets we have the fish we want to edit. As you can see, there are a number of things we can change about it, from it's lock state, nickname, color, rarity, passive income, etc etc etc.

For the record, I have not played with every way you can edit fish, I have only really tried rarity and color. Color works, but rarity seems to simply break the fish, as each specific fish is only programmed to be 1 specific rarity.
In our case, we want to change the color to golden, so I am going to find "color":0 and change the number to the color we want, in this case, gold is number 2, so I change the 0 to a 1 as seen above. (regular = 0, painted = 1, golden = 2, rainbow = 3)

Now save the file to your computer and paste it into the V7 save folder replacing the current file in there.

Once you've replaced the save file in the V7 folder with the modified save file, launch the game back up and find your new fish.

Here is the example fish in freshwater 2 from earlier, now golden!

I know this is pretty cheat-y and may be frowned upon by some but I don't really care. In this case, this one specific fish as eluded me for thousands of packs at this point, I've spent hours opening these rivers and ponds packs and have lost the interest in it. I just want to open some new packs frankly! Video games are for fun and however you have fun is the best way to play them.

Thank you for your time, hopefully somebody else is able to find this useful!