6 ft by 20 ft is not enough information for us to answer
We need to know the radius or diameter of the tank and its height and you have to tell us which measurement is which.
141.06 gallons
700 gallons
If this is a round tank, 15 feet wide and 7 feet tall, there would be 1237 gallons.
Up to 149.61 gallons.
A 20 Gallon tank can hold up to 20 gallons of water.
This tank will hold a maximum of 10,030 gallons of water.
141.06 gallons
1056 GALLONS
700 gallons
1536
If this is a round tank, 15 feet wide and 7 feet tall, there would be 1237 gallons.
A round tank 36 inches wide and 10 feet deep holds 528 gallons.
This tank will hold up to about 4,858(4,857.979293) gallons.
This size tank can hold up to 7,779.74 US gallons of water.
Up to 149.61 gallons.
209.52 gallons. Lx Wx H /231 = GALLONS
About 157.1 gallons.