Why are all the zpools named "tank"?

I was confused by this at the beginning as well.

Since the ZFS is referring to 'Storage pools', the author created the nickname 'Tank' as in a 'Tank of water' or a 'Fish tank'. It is a bit of a play on words since the English words 'Pool' and 'Tank' both refer to large containers of water. Some people find it confusing at first.

Here is an old example from the Sun Solaris 11 documentation from 2004:

Create a ZFS storage pool.

The following example illustrates how to create a simple mirrored storage pool named tank and a ZFS file system named tank in one command. Assume that the whole disks /dev/dsk/c1t0d0 and /dev/dsk/c2t0d0 are available for use.

# zpool create tank mirror c1t0d0 c2t0d0

The term is not referring to a 'Tank' like a Battle Tank, or the term 'Tank' in gaming.

If I find time, I can dig up the authoritative source of the person who created that term. I believe the term was coined by Jeff Bonwick, Team Lead for the ZFS team while at Sun.

Regarding tank - at the OpenZFS Developer Summit 2015 it was stated that they were fans of the Matrix, and it referred to the character "Tank". If you look through the documentation, right along side "Tank" are pools named "Dozer". Funny how the nerd shines through.