I am also speaking from first hand experience
As the post is in here, we would be talking from a fish pic perspective. A long zoom, say 70-300 is less use to you than a 35-80 for aquarium work. So straight away you are likely to be looking at a lense that is faster anyway. Therefore I would recommend the faster lense as it is more suitable for the job at hand from a focal length perspective. A remote flash will help tremendously for closeup/macro work, but is very limited or no use for full tank shots (especially big tanks) if you are stood away from the tank. In those cases, the "faster" the lense the better, to help avoid the need for a flash altogether. DOF will be sufficient when you are not right on top of the subject.