You can add columns at will (I.E., duplicate the ones that are there already so you can add data for multiple tanks). However, if you do, you'll need to adjust the name assignments for each column so that the chart renders correctly. This method would require the most work, but would keep all data on the same sheet.
Alternately, you could duplicate the sheet so that each tank has a separate sheet. This would keep things from getting confusing. There would still be work involved on the chart, though, as you would have to alter the dynamic names for each chart to reflect the proper sheet/data set.
Both these methods would work, but both would require manual editing for the desired result. The easiest way to keep multiple tanks would be to have multiple copies of the same Workbook, but with different names (I.E., Test Results 1.xls, Test Results 2.xls., etc.).
Personally, I'd go with option 2, but I won't pretend that wouldn't involve some work.