Once you put water in a waterbed there's no need to change the water in the waterbed until you decide to move the waterbed.
You only need to replace the water in the water bed when you decide to move it after you drain the water to move the waterbed to another home or location.
My mother has a waterbed that has the water still in it from 20 years ago and never has changed it.
The water has not turned moldy or bad and it's perfectly okay to not change the water in the water bed.
So you only need to fill the water bed once and leave it alone until you decide to drain it and move it.
Even though you don't need to change the water in the waterbed you still should and need to add some waterbed conditioner once per year every year.