Newbie with question about filling the water tank

One thing I noticed when we fill our FW tank is if we use a heavy flow it will act like it's full and start to spit out of overflow tube well before it is completely full. We now take our time and use a slower fill rate that allows us to get a max fill in most cases. We also installed an Oxygenics showerhead purchased from CW that uses less water than our stock showerhead while producing a more powerful spray.