It is not clear from your brief description
how, and at what frequency, you are intending to
do these tests.
Assuming you are using an open
top plexiglass tank and measuring the Brewster angle,
you will certainly need good microwave absorber
to ensure a good depth of null as the Brewster Angle
is reached. I only ever did water measurements in
a closed transmission line/waveguide setup, but
a colleague at Uni had endless problems with spurious
reflections at X Band using an open radiator free
space method, until we found good RAM absorber.
Three tanks should be OK, but you will need
to rinse them out for each measurement to avoid
any contaminant film across the top surface.
But maybe I don't understand your setup clearly