Help on a project

Hi there.
I am doing this for the first time and need to make a project whereby I am controlling the volume of water in a tank between 20% and 75% capacity. I need to be able to quickly change between what the capacity of the tank is and the volume needs to be independent of the flow rate.
Any help/guidance/parts I need would be much appreciated.

Take a look at the inside of a toilet tank, your decription matches what occurs there pretty well - and no electronics needed. Float & valve are available at hardware stores too.

Maybe give more detail as to what you are trying to do. What exactly are you wanting to control? What hardware do you THINK you need? Etc.

XY problem?