That is active.

Something passive could be submerging the tank in a pool of water (also good for proving spill containment won't leak).

Typically you don't have enough surface area for that. The walls are thick enough that thermal conductivity into an ambient-temperature liquid alone is not going to be sufficient.

Uh, you can't just disconnect a pressurized 35,000 gallon tank and drop it into a an enormous pool you just keep full under it at all times.