If the supply tube is rigid copper, also pick up a flexible plastic supply
tube in case you can't get a tight seal between the copper supply tube and
the new fill valve. Return it if you don't need it.-

flexible lines, the stainless braid type can fail internally, the
inner rubber hose delaminates or comes lose and buckles under high
flow, so flow starts good then drops a LOT.

I have had this occur on my toilet line, my handheld shower, and
finally my vans front brake. in the brakes case the line started
acting like a check valve, one side would drag and wear out the pads
and overheat the rotor.........