def insert_nulls(df, fill_val=-99999999.99):
    """replaces fill_val with null in all columns of df.

    :df: Dataframe
    :fill_val: fill value to be replaced with nulls. 
        default=-99999999.99
    :returns: Dataframe with fill_val replaced with nulls

    """

    for col in df.columns.tolist():
        df.ix[df[col] == fill_val, col] = None

    return df