Ever since the 3.0.0 update, the game no longer provides an Exclusive Fullscreen mode.
The graphic options we are given in game are Windowed and Windowed Fullscreen modes, pick your poison.

Windowed Fullscreen is not the same as Exclusive Fullscreen, the only advantage that Windowed Fullscreen has over Exclusive Fullscreen is that it helps your game alt-tab in and out faster, that's it. You invariably lose processing power because more of it is divided to keep the background processes running at full speed, prioritising the speed of switching tasks. Also in Windowed Fullscreen, you lose the ability to change the resolution of the game, this mode locks the resolution of the game to your set desktop resolution.

Some people may want to lower the resolution as a way to optimise the game and to keep the game Fullscreen at the same time, this is where Exclusive Fullscreen mode comes in, a basic feature that almost every game has, yet the developers of this game seemed to have neglected it for whatever reason in the 3.0.0 update and have yet to add the feature, maybe they won't, who knows.

The configurations I've made are what helped me achieve Exclusive Fullscreen.
Through sheer frustration and experimentation in getting Exclusive Fullscreen, a few things were changed, perhaps not every change contributes to the effect of Exclusive Fullscreen, but as it stands, it works. The following steps described are what changes need to be made.

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Navigate to the folder of the config location in AppData, Local and "SpaceFish".
In "This PC", then "Local Disk (C:)", Users, the name of your user, and so on. You might need to enable "hidden files" in the view options of your file explorer to see the "AppData" folder and its contents.

Inside the "SpaceFish" folder, navigate to the file in quesiton.

Open the file GameUserSettings.ini, make sure the .ini file does not have Read-only enabled so that you can edit the file and save it (By default it is disabled when you right click the file and select properties and in attributes in the general tab).

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All the values high-lighted in yellow are what needs to be changed.

• Every line relating to the resolution of X and Y needs to have its respective value changed to the X and Y of your desktop resolution (We will set the desired resolution elsewhere, step 6 shows how).
  
  
• Every line relating to the resolution mode needs its value changed to 0. For the Unreal Engine this game uses, 0 is Exclusive Fullscreen, 1 is Windowed Fullscreen, and 2 is Windowed (I might be wrong).
  
  
• And the line where is says
  bUseDesiredScreenHeight=True
  make sure its value is set to true.

The game however doesn't have the option Exclusive Fullscreen listed in its video options, so selecting 0 will show up as windowed selected in game.

Other changes can be made here before you save it such as changing FPS or Vsync, make these changes now before the next step.

Resolution quality should be kept at 100.000000, in case it wasn't.

The image below shows my settings for the .ini file, my resolution is 1440 by 1080 but your desktop resolution may be different.

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After you've made your changes, ctrl + s to save the .ini file, then close the .ini file.

Right click GameUserSettings.ini and select properties in the drop down.
In the general tab which is selected by deafult, make sure you enable Read-only so that the .ini file doesn't get changed when you run the game.



And that's it for the .ini file!
Now onto step 4.

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• Right click the game listed in your steam library.
  
• Then select properties in the drop down.
  
• Select "Installed Files" on the column to the left.
  
• Then click browse as seen in the image below.

You should see this folder with the SpaceFish.exe as seen selected in the image below.

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Right click SpaceFish.exe from step 4 and select properties in the drop down.

Select the "Compatibility" tab.
Then select "Change settings for all users" (Unsure if this is required but I went scorched earth here.)



Make sure "Disable full-screen optimisations" is enabled.
Then select "Change high DPI settings".



Where it says "High DPI scaling override" make sure the option is enabled and that "application" is selected.



That's is for the .exe! Click apply and ok for everything and close it.

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Similar to the start of step 4, right click the game in your steam library and click properties.

In the general tab, you might not need to do this but I've disabled "Steam Overlay while in-game" for the purpose of additional optimisation, it might be negligible.

For the "Launch Options" which is the most important step to make the game Exclusive Fullscreen, these are the relevant commands required:


Now replace 800 and 600 with your desired resolution that you would like the game to be.

Note, there is a space between "w" and the number and "h" likewise. "-w" refers to width and "-h" refers to height.

I've set mine to be 800 by 600 because of how poorly optimised this game is, combined with 60 FPS, my GPU won't struggle and melt.

Note, my desktop resolution has a ratio of 4:3 (1440 by 1080 as set in the .ini file.) The resolution you set in the steam launch commands for the game should ideally be the same ratio (800 by 600 is a 4:3 ratio).

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Make sure Direct3D 12 is enabled.
And if you have upscaler set to default, make sure resolution scale is set to 100 as mentioned in step 2 (sg.ResolutionQuality).

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Well, I hope this guide helped.
Maybe the developers of this game will add the much required and basic feature that is Exclusive Fullscreen.

There also may be some spelling errors, sorry in advance, I will fix them if found.

Also sorry for the very verbose guide, I wanted to cater to every type of user and that may include users who aren't very computer savvy. I'm sure you are savvy.