This is extremely low-resolution, so you don't have much room to convey a specific (strong) light source.
Note that you can omit an outline entirely, if the background is contrasting enough. Typically, I'd try to avoid dark outlines between an object and the ground, except when the object actually has tiny legs and is casting a shadow (ex: armchair)
Games do use light sources, both for aesthetics, and in order to help the viewer's mind to correctly interpret what he's seeing.
For example these tiles:
https://opengameart.org/sites/default/files/iso-64x64-building_2.pngThis follows the rule tha on concave objects, right part is lighter, and left part is darker. It really helps getting a "3D" feeling, but it comes with a price : tiles can't be mirrored, they have to be redrawn with the opposite lighting.
If the blue thing is an aquarium, I think you should rather focus on the 'blue shiney" part. Try with a lighter diagonal "slash" maybe.