Let's do this in feet. There are 3 feet in a yard so the tank volume s $30 \times 30 \times 2.5 = 2250$ cubic feet. To convert this to gallons I have to know how many gallons there are in a cubic foot and I don't rremember what that is. I could look it up but I can also internet by typing How many gallons are there in a cubic foot? into a search engine or ask Google or Siri. I'm a Mac user so I used Siri
Me: Hey Siri, how many gallons are there in a cubic foot?
Siri: One cubic foot is equivalent to about 7.48 gallons.
Hence the volume of the container is abut $2250 \times 7.48$ gallons.
You could save yourself a little arithmetic by asking Siri How many gallons are there in 2250 cubic feet?