Ag Tanks = The large, usually plastic tanks that ag chemicals AND water
are stored in. Come in assorted shapes and sizes. Some up to 2000 gallons.
(Best on a firm foundation 'cause that is roughly 16000 pounds of water = 8
tons) Cost is reasonable. Better even for the larger tanks holding capacity.
Many are also listed for "potable water".
That said - BUY NEW!!! Only those of much greater powers KNOW for sure
what has been stored in those tanks! If one even worries a tad about stuff
leaching from the plastic itself, don't even dream of what else could leach
from a used tank!
Get one with a "human size" access top. Those things will occassionally
need cleaning out. Because of their resistance to chemicals, they can also
be partially burried which will help prevent the water from freezing in
moderate winter areas (tenzicut, you don't count LOL) and also from becoming
bath water hot in the summer. (Boiled veggies before even leaving the
garden).
"Stock Tanks" = open topped tanks for watering livestock. For those that
find used galvanized ones at farm sales - there ARE plastic/pvc liners made
for most of the standard sizes.

Water capacity: I know many of you only have small gardens, but still,
the amount of water needed for a food producing garden can be amazing.
Mulching for sure helps!! So does spot watering, but for arguements sake,
lets look at a plain old open garden. Most plants need about 1" or
equivalent of rain fall per week. Each square foot of garden will need 144
cubic inches of water (231 cu inches in a gallon) or roughly a lttle less
then 2/3 of a gallon every week. On a 4' X 4' mini-garden that is a mere 10
gallons. BUT for those of us trying to grow enough to feed a family it can
get to be quite a bit. I have a growing area of roughly 60' X 80' most of it
in beds, but if I was to overhead water the entire thing that would be
almost 3000 gallons of water!!! For ONE week!! That is why farmers pray for
rain during the growing seasons and gardeners opt for mulching and drip
irrigation. <VBG>
Conversly a 20' X 30 ' section of roof can deliver almost 375 gallons of
water in a 1" down pour! Get the buckets and barrels out!

