What age do you think its appropriate to explain death? and how have you done so? ie, what words do you use?

Our beloved fish has passed on and our toddler keeps going to the tank and asking where he is. I'm not sure what to say, whether i should make something up? (ie, he went back to the ocean) or whether i should try to explain what happened. (he actually died of old age).

Let me know if you have any ideas thanks