Let the time taken to fill the pool using the larger hose alone be t minutes.
Then the smaller hose used alone will take 3t minutes to fill the pool.
When the hoses are used together for one minute, 1/30 th of the pool volume will be added. Now we can write the following equation:

This simplifies to:

3t = 120
t = 40 minutes.
Therefore the smaller hose will take 3 * 40 = 120 minutes to fill the hose by itself.
The answer is: 120 minutes or 2 hours.