I think the joke is that, the tank that humans drive in war, thats what the fish is referring to, but its actually a fish tank. I hope you understand this

It is a pun on the word "tank." A fish "tank" is a glass container or aquarium in which you can watch fish swimming. If someone says "there are two fish in a tank" that is what you think of. In English, a "tank" is also am armored military vehicle with big "tracks" instead of wheels, and a gun at the top, able to drive over rough terrain. So we realize at the end that the two fish are not in a normal "fish tank," they are in a military tank. I believe the explanation of the word "tank" is that it was actually a code word, when tanks were new and were military secrets. The British army called them "tanks" because they were not tanks (that is, they were not vessels for holding liquid).