yes but they also usually hook up to fire hydrants to have enough time to put fires out. I don't have figures but from what I remember most fire trucks only have enough water on board for the first 5-10 minutes of spraying.

Only enough for a very short amount of time, basically the water in the truck or fire engine lasts long enough for the firefighters to get connected to a hydrant and not much more.

Ah ok. I think I greatly overestimated how long those tanks last for.