If you created a tablespace with a data file a month ago, now 80% of the data file is used, you should add another data file to the tablespace. This can be done by using the ALTER TABLESPACE ... ADD DATAFILE statement. See the following sample script:

SQL> connect HR/globalguideline

SQL> CREATE TABLESPACE my_space

2 DATAFILE '/temp/my_space.dbf' SIZE 10M;

Tablespace created.

SQL> ALTER TABLESPACE my_space

2 DATAFILE '/temp/my_space_2.dbf' SIZE 5M;

Tablespace altered.

SQL> SELECT TABLESPACE_NAME, FILE_NAME, BYTES

2 FROM DBA_DATA_FILES;

TABLESPACE_NAME FILE_NAME      BYTES

--------------- --------------------------------- ---------

USERS   C:ORACLEXEORADATAXEUSERS.DBF 104857600

SYSAUX  C:ORACLEXEORADATAXESYSAUX.DBF 461373440

UNDO  C:ORACLEXEORADATAXEUNDO.DBF 94371840

SYSTEM  C:ORACLEXEORADATAXESYSTEM.DBF 356515840

MY_SPACE  C:TEMPMY_SPACE.DBF   10485760

MY_SPACE  C:TEMPMY_SPACE_2.DBF   5242880