[Tagging] wall and block that aren't a barrier

[marc marc]

Hello,

a mapper map small fountain with polygon
https://www.openstreetmap.org/browse/relation/8322503
I have nothing against the idea even if I would have used a node for 
such a small object

Of course amenity=fountain must be moved to the MP relation with 2 
polygons in stead of a invalid one.
But there are several questions about tags

- the concrete structure is currently filled by a retaining wall
barrier=retaining_wall
https://wiki.openstreetmap.org/wiki/Tag:barrier=retaining_wall
according to osm wiki and wikipedia, a retaining wall is very fair away 
of a small tank containing water
A barrier is a physical structure which blocks or impedes movement,
this is not the case of a tank
Hor how to map this small tank ?
landuse=reservoir + content=water + material=concrete ?

- the stone block on the left where the water comes from
how to map it ?
man_made=pillar ? but it's obsoleted
fountain:support=stone like it's done for bridge:support ?
tourism=artwork + artwork_type=statue + material=ston ?

Regards,
Marc