why does my tortoise sink when I put him in the fish tank?

Because tortoises are not an amphibious animal

tortoises CANNOT swim.

Tortoises can't swim, that's why

cause the water probably makes him sink

c: I dunno. Check if it's dead. c:
Oh yeah, perhaps using my common sense. I believe that the tortoise's shell is heavy....so the weight pulls it down...?