In addition, that is one of two 50-gal water tanks and the other one is pushing nine years old… so I’m also wondering if it might also be better to just replace them both at once.
May I ask why you are using two tanks rather than one appropriately sized tank?
Way too often, the builder wants to save a few bucks and installs two commodity sized water heaters rather than one water heater properly sized for the installation. Moreover, the economy minded (not energy efficiency minded) builder may have plumbed them inappropriately from the start. If your heaters are plumbed using reverse return plumbing that is good, if not you are paying for more standby loss than required and you are not getting the heated water volume that you might think you are.
The short answer is use _one_ tank for your hot water needs not two. Higher initial cost, lower life cycle cost.