index rebuild to retain performance

Below is a scenario and request input on how to retain the current performance of the system.

With change in code that need to implemented in a production envrionment, an application gets a large response from an interacting device and as a result, the applicaion needs to populate 500 rows each in two tables Table1 and Table2 out of several other tables. The percentage increase in number of rows of Table1 is projected to be 150% while for the Table2, it is 50% (which translate to daily increase of 25 million rows from present 10 million rows for Table1 and increase of 43 million rows from present 28 million rows for Table2). There is a historical data