I see both man_made=storage_tank and building=storage_tank on the wiki, but it’s not entirely clear to me what the difference between the two is, ie I don’t exactly understand when one is used over the other, or even both together.
As an example, how would the features in the provided images be tagged:
If the building was built as a storage tank (building type), then tag building=storage_tank. If it is also used as a storage tank, additionally tag it as man_made=storage_tank.
One is the type of building (building=storage_tank), the other is its use (man_made=storage_tank).
A building that was built as a storage tank.
If the storage tank is still used for storage, add man_made=storage_tank.
Source: https://wiki.openstreetmap.org/wiki/Tag:building=storage_tank
Another example:
A building that was built as a church but is now used as a theater would be building=church + amenity=theatre.
Hi most storage Tanks are not a building , so building=storage_tank is just wrong. 
Indeed the third line of the wiki page quoted above is:
Storage tanks are not always classified as buildings, and the use of building=storage_tank is therefore sometimes discouraged
Usage of a building tag for these does seem very mapper-dependent. I personally don’t, but a significant minority do use a building tag for them.
I prefer to tag silo of which I see many on animal farms which feed poultry, pigs, goat, cattle semi automatically, content=feed mostly (ATIL). Storage tank I prefer to use on bigger, circumference fully on the ground, presets good for confusing, the building=silo a classic.
There’s the old gas storage tanks with a dome like roof working like a piston cap. Don’t ask me what they’re called. Melters generated lots of gas which then went into these storage tanks, which I now know after a duck duck visit were called ‘floating roof tanks’ in englese. Could not find any in TagInfo so guess they’re truly extinct, maybe some relics still standing on abandoned industrial plots (checked ENI at Ortona… the aerials suggest there’s a few there and in use for something)
end rant
Colloquially, in English towns they were called gasometers or less often just “gas holder”. They’re less common now, but some still exist. One end of the Oval cricket ground is sometimes still referred to as the “gasometer end”.
Tanks are surely not buildings in the common sense and the same applies for silos as well.
I believe the main reason to tag them as building is the rendering in carto, because without that you will only see a tiny icon in the max. zoom levels without the outline of the object which is really poor when you think about storage tanks with dia 50m and more.
Indeed (that’s an issue from 2019).
Since having been caught with the pronunciation of ‘caveat’… per Cambridge
UK/ɡæsˈɒm.ɪ.tər/
US/ɡæsˈɑː.mə.t̬ɚ/
The Toscani like to roll the R’s, at least that’s what the assistant in the office did. 
