I think some of us have lost touch with some of the basic immutable laws of mother nature (thermodynamics) when we talk of replacing a radiator with a large resevoir or fish tank.
The CPU puts energy into our cooling system at some rate. Unless this energy is being removed from the cooling system at the same rate it is going in, the temperature of the system will increase. There is nothing we can do to change this.
The fish tank represents a large mass of water. The glass sides have lousy heat transfer properties so, practically speaking it is a lousy radiator. Probably most heat is transfered out of the tank by evaporation but even this will be very limited if the tank is covered.
Since the CPU is putting energy into the system and the system (fish tank) is not doing a good job of rejecting this energy, its temperature must increase. Because the tank has a large mass of water, the rate of temperature increase will be low. Even though the tank is a lousy radiator, it is not perfectly insulated so some heat is rejected. The rate of this rejection is a function of the temperature of the tank. The higher the temperature of the tank compared to the ambient temperature surrounding it, the faster it will transfer energy to the surrounding air. Eventually the tank will get hot enough so an equilibrium is reached and the tank is rejecting as much heat as the CPU is putting in. Unfortunately, this equilibrium will only be reached at uncomfortably elevated temperatures because of the terrible heat transfer characteristics of the fish tank.
What we call a radiator is really just a heat exchanger that is relatively small and light wieght. By design and materials it efficiently transfers energy to the surrounding air. The fish tank is large, bulky, heavy, and does a terrible job of transfering or radiating energy to the surrounding air. It will do so but only at an elevated temperature will it transfer a useful amount of heat. It does represent a large thermal surge mass so it can absorb a large amount of energy before its temperature starts to noticeably rise. However, it will still eventually take on the role of a radiator.
I hope you can see my point. The fish tank is still a radiator. It is just a large unwieldy device that will not work as well as a device specifically intended to transfer energy.