Rocket Stove concrete vs cob?

I am building a small fish house for my fish tanks. I am planning on using a rock stove for heat and want to run it through the floor under the platform the tanks will be sitting on. I have read a lot about using rocket stoves and all that I have looked at the builder is using cob. Is there a reason I should choose cob over cement? the tank will NOT sit directly on it. Thanks for your time!!

Cost and durability.
Concrete will fail at certain temperatures as the water is driven from it. Cob is just clay soil and maybe straw.
Heat makes it tougher, and of course it is cheaper.

that's good to know I was considering building my RMH with a concrete bench but I guess it will be cob.

Thank you William, that was good to know.

You could use concrete at the end of your bench after the temperatures have dropped.