Check the earning history screen to verify that the redemption has been completed (gems have been subtracted).
If additional tanks are not reflected in your account
Additional tanks are added to the right side of the tanks you originally had, and are displayed by tapping the area where the tank is displayed and sliding it sideways.
Please note that sliding operations may not be possible on some devices, such as some of those manufactured by LG, due to device-specific issues.
Even if sliding doesn't work, the tanks that you have redeemed gems for still exist. Please check how many tanks you have filled by looking at the "You can earn XXX times more" indicator below the "Can earn 1 more times" button.
If other items are not reflected in your account
Exit the app from the App Switcher (Task List) and then restart the app.