Great idea and work. 
I did a similar thing by making a box that sits on the floor in front of the fridge, my wife also had trouble seeing in the freezer section of the fridge.  The box also serves as storage for shoes, can drinks or what ever. That formed the first step, then the next step was already built over the water tank like yours. From there it was easier to climb up and we use the corner of the sink for another step if necessary.   Now to get down....when your over 80......