The first formula is an array formula, so you should commit it using
CTRL-SHIFT-ENTER rather than the normal ENTER. If you do this correctly
then Excel will wrap curly braces { } around the formula - you should
not type these yourself. I would also suggest replacing the " " with 0
in both formulae.

I presume tank1 is a named range - it should cover the same number of
cells that your date range covers.

Hope this helps.

Pete