What causes "softness" in hydraulic disc brake systems? I have a simple Shimano hydraulic disc brake system (don't know the model).

Over time, the feel of brakes is getting soft, I have tried to adjust by tightening the screw (on the joining of brake line). I have also recently changed the brakes pads myself.

Is softness due to a decrease in fluid level?
You should bleed the system with mineral oil, not top up. If it's your first time you shoud see a tutorial video first on utube . if there is no leak you don't need to tight anything. if you changed the pads you must brake at least 50 times on a safe place at 25 km/h to get the full braking power.