I will assume that only one history record will be created on a given date for a specific tank and that your relationship between Tanks and Production History is:
Tanks::__pkTankID = ProductionHistory::_fkTankID
If so, you can add another Tutorial: What are Table Occurrences? of ProductionHistory, name it "ProductionHistory|Previous" and link it like this:
ProductionHistory::_fkTankID = ProductionHistory|Previous::_fkTankID AND
ProductionHistory::Gauge Date > ProductionHistory|Previous::Gauge Date
Double click the relationship line between these two occurrences and specify a sort order for ProductionHistory|Previous that sorts the records by Gauge Date in descending order. (Gauge Date must be a field of type date, not text.)
Now you can add a date and a Gauge Reading field that auto-enter these values from the original date and Gauge Reading fields but from ProductionHistory|Previous to copy over the previous readings and date so that you can subtract the dates to get elapsed days and subtract the gauge readings to get the difference in order to divide the difference by the elapsed days.
Thanks Phil. Yes, only 1 per day.