From OpenStreetMap Wiki
|man_made = water_tank|
|Using this tag is discouraged, use man_made=storage_tank + content=water instead.|
|Used on these elements|
|Tools for this tag|
|This feature has been labeled as deprecated. The recommended replacement is: man_made=storage_tank + content=water. The reason is documented under Deprecated features. You are still free to continue to use or interpret this tag as you see fit since OpenStreetMap does not have "banned features".
Under no circumstances should you (semi-)automatically change "deprecated" tags to something else in the database on a large scale without conforming to the Automated Edits code of conduct.